If using a fixed speed pump sized for the system flow rate but divided into multiple zones, when all zones are calling the pump and pressure will meet the design. When the zones start to close the pump cannot adjust so pressures may go up, flow rates or velocity may increase.   That is the purpose of the differnential bypass valve to help bypass the zones and keep pressures equal or closer to equal.   With the technology of variable speed pumps they can auto adapt to help maintain proper flow, pressure and velocity with changing zone demands.