I just pressed Brain-AFK banish, instead of challenge. My own fault, yes. But that made me think, for what is the banish button anyway? I mean, if you want a new set of items you can just challenge the nemesis. And if you dont want to fight it, you can just move away.
I would like an confirmation window after pressing banish, to prevent me being stupid the next time 
If you don’t fight it, it will have the same set of items the next time you find it.
Banish is for when none of the items are worth it and you don’t want to waste time fighting him. That way you’ll get new items on your next encounter.
Basically: banish = challenge - fight
If you banish, you don’t have to fight it and the items from the next Nemesis will be different. If you think the items it has are trash and don’t want to fight it to get them or empower them, then banish it and go on.