If you have too few runs it restricts the amount that can get to an amp. (a run of 1/0 can only carry SO much power... right?)
Adding more runs means you can get more power from one end of your vehicle to the other.
If you think of it in terms of trying to pass water through a hose... more/larger hoses mean more water can get from one end to the other
This is also why it's important to have as many positive runs as negatives. You need to have proper 'flow' in both directions.