The BrawnBoost implant acts to circumvent many of the legal restrictions on athletic pharmaceuticals by avoiding any actual drugs, and simply stimulating the musculature with electric and magnetic impulses.
In addition to expanding your inventory and armor-equipping capabilities, BrawnBoost implants make you pretty fearsome when swinging a Wrench. Be careful though; if your inventory is too full when the BrawnBoost runs out of charge, you'll automatically drop some items.
You can also use this implant to meet minimum Strength requirements for some weapons. This is risky, however, as the weapon will automatically un-equip when the charge runs out! Swapping one BrawnBoost implant out for another will momentarily lower your strength, and also cause you to un-equip strength dependent items.