My jazz group would just take any paper sheets and either re-transcribe them or take photos/scans, send em to Dropbox, and then read them in forScore. Our whole repertoire was in Dropbox in case we had somebody sit in on a gig who needed to play lead lines or something. With Scribd and digital realbook collections, the amount of music that’s around digitally is mind blowing.
As for why an iPad, I think it’s more that either folks already have iPads, or they get them because “creatives” like using apple products. Same reason musicians will often get a MacBook instead of a dell even if they don’t need it for any particular reason. Both of those reasons would lead to the network effect of folks asking their friends/colleagues what they use, and then buying an iPad because of that.
Big-band music is sadly in much worse shape for digital preservation. Anywhere from 16 to 19 parts per chart, 2 to 5 pages per part. A few band members have been individually digitizing their "book," but there hasn't been a coordinate effort.
My other band, a quartet, is more like what you describe. And anything new is going to be in digital format.
As for why an iPad, I think it’s more that either folks already have iPads, or they get them because “creatives” like using apple products. Same reason musicians will often get a MacBook instead of a dell even if they don’t need it for any particular reason. Both of those reasons would lead to the network effect of folks asking their friends/colleagues what they use, and then buying an iPad because of that.