What is so great with biscuit joinery?
That's all I've got, because other than that I don't think it's a good joinery method, for finer work especially.
So at school we are doing a bunch of different joints and biscuit was one of them.
Biscuits aren't a joint per se as they're a reinforcement or alignment aid. Many butt joints can be reinforced with biscuits, including mitres, standard 90° butts and edge joints. Arguably the first two are rightly called "biscuit joints", but in the third the biscuits are there as an aid to alignment during the glue-up (and therefore have little real value, they certainly don't add strength as some believe and can in fact undermine the joint in the long term, also see reference to telegraphing in this previous Answer.).
I also think that dowels are stronger, and here are my reasons. Dowels fit tightly, while a biscuit is rather lose
Dowels can be stronger, but it's all about the specifics. The diameter of the dowelling used, their overall length and how deeply they are seated in both workpieces are all significant factors. And it goes without saying that the species of wood used for the dowel is important.
Re. the loose fit of biscuits, they're supposed to only be that way when first inserted. They're made of compressed wood (beech usually AFAIK) and they are intended to swell to a tight fit after the glue is applied. This is why biscuits are supposed to be carefully stored to keep them dry. It's also why you have to be mindful of the glue used, any glue that doesn't contain water is not as good a choice as one that does.
seems to need glue filled in more or less
For most woodworking you should never think of glue as a filling material or gap-filler. Most common glues are truly terrible gap-fillers, relying instead of very close fitting of components to make a strong joint (the thinner the glue line the better, as odd as that might seem).
In addition to this general point you should be careful not to over-fill a biscuit pocket with glue. Apart from the waste in glue — most will simply slop out as the biscuit is pushed home — if you make the pockets overly wet you can get localised swelling, which is a problem more than a few users have discovered the hard way.