Yeah, an Nfet needs a positive Vgs so it needs a voltage above the S pin. The "high side" mosfet is often switching the highest voltage in the system so some sort of voltage boost circuit is needed to generate something ~10V higher than that. The bootstrap cap charges up to around 10V above the S when S is at ground and is used as a voltage reservoir to maintain that +10V even as the mosfet turns on and S rapidly rises.