I'm pretty sure clicking the unsubscribe button says "hey, this email address you obtained is real and active!"
That feels very "20 years ago" to me. They've got plenty of other ways. Don't get me wrong, I'm a bit careful with them but because I'm used to being signed up to lists by a handful of stupid people who don't know their email address, I can basically anticipate what regions of the world my spamlists will come from, and I generally have a good feel for what's legit or what's not. I use the unsubscribe link often for these things.
Many less technical folks do. If I never signed up, I'm absolutely not clicking an unsubscribe link.