The only tangible benefit I ever received was two small checks after performing at some high end benefit concerts.
In the early seventies I joined a band that had a high visibility house gig at a club in a strong Union town. The second night I played, the local union president was at the club to invite (read: command) me to the Union hall the next day, to get signed up (LOL). I maintained my card into the mid eighties, but decided to let my membership lapse since I had moved back to my home town, and wasn't doing any union gigs.
I started doing regular recording sessions though, and twice had to renew my membership in order to play on records. The last time was in '98 while doing an album on a major label (Mercury). The artist's manager asked me for my union info, and seemed perplexed when I told him I was inactive. He told me it wasn't mandatory, but i wouldn't be credited on the album and wouldn't benefit from any future royalties, etc. I then got on the phone with my local's business mgr. who said he'd put a card in the mail if I would put a check in the mail!
I've gone inactive since then, and probably won't have the need to ever join again, because my health issues prevent me from doing any major projects anymore.