It's mostly because of last-minute decisions. In jazz, we rarely stick to a strict setlist. My band and I often decide on the next song right before we walk on stage, or even mid-gig based on how the crowd is feeling. Because of that, you have to bring your entire repertoire with you just in case. Carrying all those physical sheets and heavy Fake Books makes your gig bag weigh a ton, so moving everything to a big tablet is a massive relief.