What it wont do:
Make your shields START charging sooner
What it will do:
Have you ever noticed how shields often start charging quickly and then slow right down? SYS is to be considered a 'reserve'. When shields start charging, if you look at SYS, it will usually start to deplete. If this reaches the bottom, shields start charging slower. You can divert power to SYS when the yellow bars are low, this is pretty much the only time it's of benefit. If your shields can charge back up without draining the yellow bars down to the bottom, you'll never need to divert power to shields, as your shield generator is already recharging shields at it's maximum speed.