I've met many fine musicians who can't read standard notation. They play by ear and/or read only chord charts or tablature. I admire their abilities as I'm pretty much crippled without sheet music in front of me, until and unless I've memorized the piece. I've spent years trying to get better at improvising, but with some exceptions for rock and jazz vocals, it just isn't in me. I really prefer to have all the notes written out. I do hope I can get beyond this need one day, as it goes counter to the message of an amazing book I read recently by Victor Wooten, The Music Lesson, which I highly recommend to all musicians, especially bass players.
But, back to written music for now. I like being able to sight read. It's great to be able to sit at the piano in front of a piece of music I've never seen before and just play it. My first paying job was actually playing for a musical theater class at age 16. I played lots of auditions. Imagine the stress of singers throwing music you've never seen before in front of you every five minutes for four hours. I did this in college too, often for free. Do me a favor, watch "How To Treat Your Pianist" if you ever expect to sing in an audition yourself.
I'm not nearly as good at sight-reading on bass or voice. Bass because I've yet to get truly confident with the instrument, voice because I don't have good relative pitch (and definitely not perfect pitch). In that regard I really appreciate that our choral director is making us do sight-reading exercises at the beginning of almost every rehearsal. There's some grumbling about it, including from me as I already understand the concepts of key signatures, pitch, and rhythm, but overall we really need it. The musical skill levels of the chorus members are all over the place. I was showing some music changes to a fellow alto who couldn't make it to our sectional rehearsal, and explained that we were to sing a particular passage an octave down. It dawned on me after a short time that she didn't know what an octave was.
In any case, it's saved me a lot of time that I've been able to learn the choral music on my own at my keyboard rather than waiting to download and listen to the rehearsal files our director's been providing. It also enabled me to participate in a very special wedding proposal gig last weekend with only one rehearsal, after receiving the music only a few days before. (Most of the other chorus members had already performed this piece.) And with eight songs in our upcoming performance to learn, of which seven need to be memorized and at least one with choreography as well, I've got my work cut out for me.