By far, the biggest bonus of a void-signature system is cutting visual spotting range to 1/4 of normal during double-blind play. Normally, you can see an enemy unit at 60 hexes. With a void system in place, that drops to 15 hexes. You can fire LRM's or PPC's while remaining invisible to the naked eye. Under low-light or inclement weather, it gets even more ridiculous. Being able to fire a broadside of SRM's into a target at night when they can't see you is just sick.
Thankfully, the void-signature system is an F-rated piece of equipment, extremely rare in the galaxy-at-large.