it have the description of "You can specify the start parameters that apply when you start the service from here". but i dont get what it does.
Basically extra commands to a program if it supports it.
For example, an application may be multi core aware but by default only uses one core, the developer may make it take up multiple cores if started with the switch /cores 2 or if a program can span to multiple GBs of memory, there may be a switch that tells it the maximum it can take up - /maxmemory 1024
It is not built in and always there "because it is a service", it is something specific to each individual service programmed by the developer.