Because focus adds, as you can see in your own post, RCT (ranged combat tactics). If you have more RCT you roll higher, which will help you defeat your oppent.
Looking to the game now, it doens't really matter where you put your points in, atleast thats my expierence. If you say your primary a gunner, you can be a ma, a coder and a hacker as well. It's ashame, but I'm sure they'll fix it later.