grounding question re: the neg battery cable

Hey there, I need help understanding how the ground from the neg battery cable affects the ignition system

-why does the neg battery cable have 2 grounds?: One to the main block and one to the chasis of the car

-if you only used a single ground to the block (like on my tractor), would this affect
the ignition system and if so why?

Trying to be smarter than the machine that's trying to beat me

Thanks so much. God bless

Keni