wiring speakers to amp?

Ive had an installer few months ago wiring up mys speakers. However, I see that he also connect OEM wires to the speakers new wires as well, whats the reason to do this? In the rear side of my car, things even looks confusing. Even though they did run a RCA signal cable converted from my OEM head unit of my Honda to the amp, so why do they need to use the OEM wiring attach to the new 16g wires of the speakers too? So is it gonna run better if i just run straight 16 gauge wires straight from the amp to the speakers rather than the set up of the installers did for my car. I kinda confused of what he did on the wiring though.

that is true, some are too lazy to run 16gauge back up to the front for the front speakers so they use the factory harness in the car to do the "double back" to the front.

oh ok... i guess its hard for him to put the 16gauge through the hole close by the door so he uses the oem wires... im gonna rewire the whole thing tomorrow with better cables.... the rear wiring is even more confusing though i have to firgure out. yes all 4 speakers are amped. Another thing is im having hissing noise in my front speakers, what might bee the cause?