Everyone has different reasons but mine are just to bypass lockouts
One character = one attempt at invincible per week
two characters = two attempts
x attempts at some stupid toy that drops from a rare in Frostfire Ridge
x alchemy xmutes a day
Anything that has a lockout benefits from having more characters
Absolutely I multibox daily but I only play 2 at a time.
For me 250 slots is enough, I have no desire to ever reach that Blizz would have to add like a dozen more new classes for that to ever not be enough so Iβm safe for a good long while