Her ult does have two big things that make it good for offense: it ignores barriers and it has massive range. I've coordinated a successful push by telling everyone to go in when I ult, and I just melt the enemy hiding behind an Orisa shield at the choke. It can even pick up a kill on a backline support, which is pretty sweet.

Damage orb is fun, but I prefer healing orb for a few reasons. First, it heals up to 300 health (as opposed to the 200 damage of the orb), which cancels out a ton of damage. Second, it actually charges Moira's ult faster, because you get 6 points of ult charge for every 5 healing you do, rather than 1 point of ult charge for every 1 damage; that's 360 potential points of ult charge vs. 200. And getting ult faster means more free healing faster. Finally, if you don't land the kill with the damage orb, it gives ult charge to the enemy healers as they heal their allies back up, whereas the healing orb can only ever give you ult charge.

It's pretty sweet to lob a healing orb out over top of your team as they advance and stay topped off even while taking damage. That said, if I can pick up a cheeky solo kill, I'll sometimes switch to damage orb if I can angle it to stay on the enemy. That, or if we really need to finish off an enemy we can't quite get to.