I like to see whole shows, not just the headliner. I particularly like it when the opening act is an exciting surprise.
I amazes me how many people skip everything before the headliner.
In the same way I like to visit the side stages at festivals. Some of my favourites have been:
Lover Boy opening for Bob Seeger - never heard of them before the show
Third Eye Blind opening for U2 - best surprise seeing how many good songs they had, having heard only one on the radio
Explosions in the Sky at the 3rd stage at V Fest -I still love this band
Holly Cole on a street stage at lunch during the Montreal Jazz festival - never knew she existed before that