What exactly is the salt for when making oatmeal porridge?
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.