You have learned to better store your equipment on your person. With each level of this Perk, the AP cost to access Inventory in combat is reduced by one.”— in-game description, Fallout
With this perk, cost to access Inventory during combat is only 2 AP.
This perk can be very valuable for combat-oriented characters. A normal weapon reload costs 2 APs, the same as accessing your inventory with this perk. That means you can reload your gun, use stimpaks, pull out a different gun and do pretty much everything else in your inventory for only 2 action points, as opposed to simply reloading via the action bar.
With this perk, costs for actions in your inventory are halved.