What exactly is the salt for when making oatmeal porridge? As per the instructions on the oatmeal package, I make it like this:

I take a bowl.
I put 1 dl oatmeal into it.
I put 2 dl water on top of it.
I put 0.65 grams of salt on top of it.
I mix them together with a spoon.
I microwave it at 750 watts for 2 minutes.
I take it out and mix it with the spoon again.
I wait for a few seconds and then apply a spoon of lingonberry &quot;jam&quot; (the kind where you can see individual berries).
I pour some milk over this and eat it.

What exactly is the salt for? Is it purely for taste, or does it actually cause some kind of chemical reaction which makes it cook properly or something?
There are several reasons to use salt in cooking. From that source, the UK Salt Association (yes, that is apparently a thing!), salt is used as:

Seasoning
Preservative
Binding agent (in meat products)
Color Controller (in meat products and baked goods)
Texture aid (in meat products, baked goods and cheese)
Fermentation controller (in baked products and cheese)

The only one of these that applies to your recipe, is seasoning.
Note that apart from tasting 'salty', salt is also a known flavour enhancer which can improve the taste of food and drinks without making them salty. This is why salt is often used even in sweet applications, and occasionally even in cocktails.