the musician unions is for supplemental things like benefits - health insurance and life insurance, some deals/discounts on certain things like from some instrument retailers, credit union membership depending on your local area, etc.

There’s little to no benefits for negotiation/collective bargaining with music labels or venues, because most musicians aren’t part of unions for them to have that power. Back in the day there was a negotiated scale rate for studio musicians who were part of the union, but that wasn’t also for lead artists who were at the mercy of the labels and those contracts.