By upgrading, you can increase your ability to suck enemy heads off, speed up your regeneration, increase your thrust and health and plenty more. (the-gamers-lounge.com)
They just gave you power ups and let you smash enemy heads open with them. (gamingreinvented.com)
For most missions I was ghost and sniper, the warrior only came in when I was using explosives and semi-automatic weapons when confronting enemies head on. (gameindustry.com)