For more guests to get a better viewing experience. Especially in large auditoria wherein it's very difficult to get the people on the edge to see the 3D as well as those in the very center of the crowd.
For more intimate settings in which the animation must "squench", flat panels are used because the focus is on a much smaller and more mobile audience. However, this is best accomplished when set pieces are combined with the animation. In the case of Forbidden Journey's domes and Spider-Man's/Transformers' curved screens, there are no set pieces to anchor those scenes (mainly because they're scenes where the scenery is constantly changing because of flight or rapid lateral movement), so they remained curved for an all-encompassing visual experience (if you look from side to side, there are images there as well).