The standard structure for private equity funds (and venture capital funds) is “2&20”, as in a 2% management fee (±1%) and 20% (±10%) of the profits, a.k.a. “carried interest“. Why 20% of the profits?
Henry Kravis of KKR explains below, starting at 6:00. TL;DR: Necessity, as he and his partners had no capital to put at risk.