>>534665256
>does that jail even do anything useful
As long as you keep the character in jail it removes him from the campagn map (you can castrate N'kari / Gorbad / Skarband's faction easily).
At the campaign start you only have 2 options for captured characters, to brainwash him or to kill him.
The 1st one will release him but you get money, view on where he is and some Eltharion's special ressource. The 2nd one will kill him (not if LL), give XP to Eltharion and some Eltharion's special ressource.
You can unlock (with the special ressource) a 3rd option to interrogate the prisoner and grant your Mistwalker units some unique bonus/ability.