What Is The Function Of The Getsimplerates() In An Iservice?

When building a custom shipping service I was under the impression that a request is made in the getSimpleRates() function and it would then pass the response from said request to the processResponse and processError functions. However after much debugging it seems the response actually comes from the getRequestData() function.

So I am looking for some clarity as to what the getSimpleRates() function does. I have taken a look at some of the shipping examples and they didnt really help me.

getSimpleRates function is used to make a request to shipping service API

getRequestData function is used to prepare data for such request (url, data. headers, etc)