so everybody is like this? why do we pin threads?
Because for some of them, if they weren't pinned, they would literally be invisible in the sense that they'd be buried under dozens of pages, never to be seen again.
As for other examples, in the Congressional/Gubernatorial boards, I've tried to pin threads that are useful to upcoming elections, such as recruitment or special election threads. I think in that case it makes it easier for people to access because their is regular chatter anyway, and hopefully in some cases the ease of access may even increase the activity
(although doubtful I think)