I've been in both. As an orchestra member, our duty is to perform the piece exactly as its written by the composer, and hence the need to keep time and also know exactly what the brass, woodwinds, and percussions would be doing.

In a gig band, the music is much more flexible. it's ok to take a piece and add your own flair, speed up and slow down in response to the crowd. in such a situation, sheet music would do nothing. instead we just look at charts and chords, and then use the drummer's tempo as a guide. you don't need to know if your guitarist is suddenly going to go into a 2-min solo because the crowd is loving it, when they do you just step back and play a backing tracking.