With all the stuff that has gone wrong at gigs over the years I take spares of most things apart from an amp (that’s more down to lack of funding though), plus I can always go through monitors and front of house at a push.
Spare cables, strings, bass, power leads, and I have multi tools, and batteries in various pockets of bags as well. Comes in handy, last gig for some reason, 3 mic stands all needed tightening at various points and I had a multi tool for the job when no one could find a screwdriver.
A good friend of mine plays in a band and they all go in ears and no backline. At some big festivals in front of crowds their Neural Pedals have gone down, DIs broken, guitars gone haywire etc. They are now taking multiples of everything.