Bonjour Joakim !

Aaaah l'éternelle question ;)

Here we're talking about a general category of collected items: I collect stamps...

If you said "Je collectionne des voitures anciennes", it would reduce the scope of items you're collecting = I collect some antique cars.

I hope that's helpful!
Bonne journée !

Bonjour Joakim !

In the case of collection, you will use the more encompassing les, because collectors' purpose is never to collect just "some" of the items :)

In general, the definite article will be less restrictive than the partitive, in that it refers to a whole category in a general manner:

J'aime le chocolat.
I love chocolate. -> the general category known as chocolate, comprising all subcategories of it

Je mange du chocolat.
I eat [some] chocolate. -> not all the chocolate that exists, but a restricted part of it!

I hope that's a bit clearer :)

Bonne journée !