Like I saw a Widow up on a ledge and I'd fly up to her, buff her for a bit so she could snipe better, then zoom back to heal tank then heal others and buff them in a fight etc etc. Then I'm all why don't other mercy's do all this other stuff?!
Probably because one of the current metas are Triple Tank Triple Heal. Healing someone boosts the healer's Ultimate charge. And sticking to a Tank means more time healing means faster Ultimate charge. However the key to the 3T3H is the Ana really due to her Ult charge.