The Champion Chamber is a defensive building, but it can't defend. Having said that, it's only use or means of defense is, as one once said, "to take damage". It will delay monsters for a few seconds, but other than that not much else.
Walling it up
I never do this.
Some people do, because they sometimes want to be able to quickly change out their champion, especially if they haven't a Korath. At earlier levels, when champions heal fast, say at levels 1 to 2 this idea seems plausible...
...unless you consider that Gorgo takes 3 hours to heal from zero to full, Drull takes 1 hour at level 1, fomor takes 1 hour at level 2 but is not worth it at level one for such great attacks, and the Champion Chamber takes just an hour to repair itself.
Alright, so your Champion didn't take much damage, and you want to switch him out pronto for another to wallop your enemy with...then that Champ comes back all dead...it's just tedious. But fine, ifyou want to do that, up to you.
Just remember that at later stages it takes days for your champs to heal, compared with one hour for the Chamber, and anyway you probably would stick with whatever Champ you had for attacks and defense. And don't tell me you will use the Zafreeti healing method on WMTs to switch out a champion - it's wasteful.
And if you really want to raise a weak Champion without getting your Champion Chamber damaged, make a strong yard!
As part of NEC
Probably all it's used for. Since your Champs don't feel a scratch when their icebox collapses on them, go ahead and leave it out in the open! Prevent your enemy from flinging monsters at that patch of open space! Delay his anything monsters!
The most useless defensive building!