Here’s how I think a GoG subscription might work out:

The money goes into a balance on your account. It just accumulates store credit.

You might raise the question of “why would anyone decide to do that”, to which I put to you this:

I want to give myself a “gaming allowance” of a little bit each month but saving is hard. If I am paying GoG a monthly subscription, though, and I accumulate a balance of credit, I can get games without it impacting my budget outside of what I’ve exactly allotted.

What else do I get? The warm fuzzy feeling of supporting GoG. Which I’d definitely do anyway. Honestly, letting me turn my subscription into store credit is a huge boon. I don’t NEED any perks beyond that.