If you get your levels and mix right, you can usually avoid feedback. But the dynamics of a gig can change and this piece of kit is there to keep your sound clean.
Especially helpful if, like me, your the sound engineer and playing at the same time.
What I really like about this Feedback Destroyer is while it's very configurable, it can also just get on with the job with minimal setting up.
I set mine up to automatically notch out any feedback frequencies before they're audible, then drop the notch after a minute or more, with two channels working independently.
Have bought a couple of these now and have recommended them to a number of friends in other bands.