>>280776311 (OP)Because he is the convenient plot device to show how someone is strong or an asspull powerup. That doesn't work if the opponent is a shitter like Shinji.
Fate route: Berserker served to show how strong Archer really is by taking some of his lives alone and to show the stupid Caliburn projection.
UBW route: Berserker is used to show how strong Gilgamesh is and how the fight would have gone differently if he wasn't nerfed by trying to protect Illya.
Heaven's Feels route: Berserker (dark) is used to show the bullshit skill copying projection.