The reason for having 8 heroes instead of 6 is simple:
You can change which 6 out of 8 u choose EVERY ROOM, but you can only change the specs of your heroes when you RETURN TO TOWN.
I.e. for something with many many rooms (like GD), those extra 2 heros means 2 extra specs u can switch between depending on what monsters you see.
and even if you don’t actually use different specs, thats still 2 extra heros u can fully feed beforehand and swap them in so you waste less item uses on feeding your heroes in the middle of tough battles.