There are seven Servants that can be summoned during the rites of the Holy Grail. While most are summoned ordinarily, the class of Berserker requires a greater sacrifice on the magus that's calling them to their side. Berserker, a Servant prized for their strength, speed, and overall improved stats come at a cost that they must be summoned with a pained reminder of the time they lost control in their lives.

In short, Mad Enhancement bolsters their abilities, but sacrifices a portion of their consciousness as well as their sanity.

[ It's very very difficult to keep one restrained. ] If he attacked you, then no doubt Lancelot will want to seek out revenge.