why DSP is required for over current numerical relays
DSP (Digital Signal Processing) could emulate the response of an existing mechanical overload protector, and thus conform to pre-existing standards. It could also allow thermal modelling of the device being protected. 

For example, this paper describes a TMS320F2812-based overcurrent protection that meets the IEC 255-3 standard. 

The benefit is to avoid false tripping or failure to trip under operating conditions. The cost is that the calculations are complex and have to be completed in a short time, so that's where DSP comes in rather than a general purpose micro. Some MCUs have DSP functionality.