They equalize fuel distribution between all cylinders which improves performance, reduces KR and improves economy.
Basically, the oxygen sensor only reads the end result from all 6 cylinders, so if one injector is flowing differently than another, the O2 sensor can't tell. The PCM can only correct for the whole.
So, if you had one injector running really rich, it would lean out your other cylinders or vice versa. Flow matching eliminates this possibility.
That'* my understanding anyway. I'm sure others can elaborate too.