I've done some very cool stuff because I can read. I'm not a great reader, but I read well enough to have been called to do 24 studio tracks arranged for a jazz big band for TV music beds. 1-2 takes and you're on to the next one - no time for being taught the part on the spot when a studio full of other musicians is ready to go.
I just did a subbing gig last week with an excellent Americana, Roots and Blues band. The first question I was asked when I got the call was "do you read?". When I got the book - just a couple days before the gig - about half the tunes had specific written lines to match the rhythms of the drums on the band's original tunes. Some of the material was sophisticated New Grass with bars of odd meters, unison bass/banjo lines, unison rhythms, etc.
I pretty much nailed every tune on the first read through. The band was impressed and very happy that I could nail the feel and sound they were after just reading their book. I had the freedom to put my own spin on the written lines, but the written lines were invaluable in helping me to play like I knew the material - and the band let me know I had done that. They told me I "fit like a glove", sounded like I'd "been in the band forever", and was "really smooth". Nice to hear when you're playing originals for the first time with pro's who've been together for years. That whole gig was on upright, by the way.
I also get to use my reading chops (which, like I said, aren't great) when I play with an orchestra + Band for big church services (Easter, Christmas Eve, etc). There's often new music that we're all seeing for the first time at the first rehearsal (usually 2 days before the event). We don't have time to fumble through all the rhythms and modulations and what not by ear. Having the rhythm chart with all the rhythms notated (not necessarily specific notes, but sometimes) means we can blast through it once and we've got it.
I've done quite a few other gigs with all kinds of people who hand me a book, usually on the gig, and expect me to read through it on the spot. One guy has a 2" binder full of everything from Elton John to Joe Sample to Frank Sinatra to George Jones to The Jackson 5. You never know what he's gonna do, but you better be ready to read the parts when he does...
Don't think for a minute that I can't improvise or learn on the spot by ear. I do that most of the time, but I've done some really fun things because I'm not totally illiterate. What I've found is that reading is a passage way to getting in with the best pros; the guys who don't have time to rehearse their 3 hour set 2 or 3 times cause they're playing every night but that one they gave up to rehearse with you.
Or you can hang out in the garage...