'Guards' is such a poor name for these. They are better described as 'noteworthy positions'. You will use the guards a lot, but really only because they are optimal positions from which to launch actions. In reality, the 'guards' do not guard you in the sense that they block attacks (although they can do!). Think of them as positions of being 'in guard', i.e. ready to do something.
LL's system of guards is very smart. Firstly, there are not very many of them. Secondly, they are used a lot because they double up as positions from which you attack from, positions which you attack to, and even positions that you attack through on your way into another position. They are also the positions you will use for binding and winding.
The gloss says very little about the guards, other than they are positions from which we fence. Dobringer says we should not hold guards for long, although the other Glosses simply says we shouldn't hold any guards other than the four positions:
High
Fool
Ox
Plough
The four positions
High (Day, Vom tag): can be held at the right shoulder or above the head. Can also be held at the left shoulder. Most attacks are launched from this guard - Oberhaus (descending cuts) and Unterhaus (rising cuts). Also, the 'secret strikes' are mostly launched from this position.
Fool (low guard): can be held in the centre or off to the side. If it is held very offcentre, it becomes a Side Guard (or The Sweeps, see below). A good position for quickly making a false edge rising strike, or quickly sticking the point out. Although not stated in the gloss, it is thought to 'fool' the opponent into believing they have an opening to attack, when in reality attacking this position is risky. The Zettel says 'Fool parries' - it is not clear whether this means that only 'fools' parry strikes, or that the Fool guard can be used to parry, or perhaps it means both.
"Who is the bigger fool? The fool or the fool who attacks the fool?" Obi Wan Kenobi (probably)
Ox: a point forward guard with the hilt raised above the head on the left or right. Note that when held on the right the arms are crossed, making this harder to do in HEMA gear. Not massively useful as a 'guard', but from this position a thrust can be made, as well as cuts. It is the natural end point of an Unterhau, and is one of the four hangen used in binding and winding. Therefore, this position will be used extensively, even if you decide never to use it as a 'guard' per se.
Plough: a low guard with the point forward and the hilt held somewhat at the hips. This is one of the four hangen and so is used a lot in binding and winding, as well as in parries. It is the natural starting position for a thrust. It is also a good parry and, by raising the sword up to breast height becomes a 'common parry' against high attacks, if needed.
However, be warned: beginners (and not so beginners) tend to hold this guard too far forward in front of the body. This is partly because of bulky HEMA gloves, but also because people face too square on to the opponent. The correct way to hold this is to turn more side on to the opponent so that you can hold your hands in such a way that they are no closer to your opponent than your head is. If you hold them in front of your body then they become a target and most opponents will try to attack your hands ('hand sniping'). If you get your hands hit a lot it is likely because of this.
Again, notice that when held on the right you need to cross your hands, which makes this harder to do with bulky HEMA gear (but easier than right Ox).
Secondary Guards
Longpoint (the speaking window): Longpoint is not a single position. It is, rather, the end position of an attack that ends with outstretched arms. This means there are broadly 5 variations of longpoint, which are the result of thrusts, oberhaus and unterhaus.
If held out as a Guard, longpoint provides good protection (temporarily), as long as it is being used to threaten the opponent. However, beware, as your hands are vulnerable, and therefore the cross guard should be turned horizontally to offer increased protection.
It can be tempting to lower your hands when in longpoint, which turns this guard into a Plough-like position, exposing your hands. Longpoint is best held with hands about level with your sternum.
Most attacks end in longpoint, and so longpoint is the position with which you will bind on your opponent's sword. Because this act of binding (which can come about because you or your opponent has parried, or because you have both willingly gotten into a bind) provides you with information about your opponent. Longpoint is therefore also called the 'speaking window'. Longpoint has been called 'the best and noblest of all guards'.
When in longpoint, the point of the sword is held towards the opponent's face or chest. In the photos above, the camera was about 1 metre off the ground, so the point looks a lot higher, but in reality is about level with where an opponent's chest or face would be.
Cuts can also be made from Longpoint by performing a 'moulinet' type motion (basically rotating the blade forward and around to make a circle), but doing so leaves you open to attack temporarily.
Barrier: this is a position from which a Krumphau (crooked cut) can be made. There isn't much more to say on this until we look at the Krumphau (you get a sense of what the Krumphau is by looking at the two images below). If raised higher it becomes a common 'hanging parry', and so I on occasion use it as a cover as I close and attack.
Side guard (Nebenhut, fencing from 'The Sweeps'): Covered in other writings from Leichtenauer's students. This is basically Fool's guard held far to the side. It is the end point of a diagonal cut from above to the ground. Its main purpose is as a position to launch a rising false edge cut (a 'sweeping cut'), which itself it used as a sort of 'universal parry'. I personal like this position and use it a lot. It possibly made its way into the system from Fiore's system, as the position is also called the Iron Gate.
Practical guidance
Since LL's system wants us to attack our opponent first, the guard we hold ourselves in when out of distance makes little difference other than these reasons:
Is the guard the optimal position from which I can launch my attack?
Does the guard (or rather the attack I can make from it) threaten the opponent or have a good chance of parrying their attack if they make it?
How does the Guard alter my opponent's expectations of what I will do next?
My experience so far is that high (right shoulder), fool, the Sweeps, and left Plough are the most useful to use as 'Guards', changing between these positions in reaction to your opponent's guard, and to keep your opponent guessing what you will do. Because of the protective gear we wear in HEMA, I tend to avoid Guards where I need to cross my arms. This means that our HEMA gear tends to prevent us (me!) from fencing in a truly historical way.
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