If you already own a game, having a steam copy allows you to do a few things..
A) Installed on multiple PCs without worry that you are running out of "activations". This probably isn't a big deal for some, but I'm a bit of a PC nerd. My computer gets upgraded every 9 months or so. Additionally, sometimes I like gaming on my home media pc downstairs on my 65" TV, so I have Big Picture mode installed downstairs. I can see Combat Mission on the big screen being pretty sweet!
Oh, also plan on installing Red Thunder to my Surface Pro 3 when it arrives, since I will be out and about over the summer, having some PC gaming with me will be a godsend. Again, easier with Steam.
Community. If I want (don't have to sign in) I advertise the games I'm playing with all my Steam friends, who are likely to be like-minded individuals. Easy way to get multiplayer games together. Steam also provides an easy way to share screenshots and the like from the games I've played so I can show off.
C) Maintenance. Much easier to maintain patches and be kept abreast of what dlc/upgrade packs are available for the games you own.
Steam isn't a requirement, and you don't have to use it, but it is pretty awesome for the games you already own, and is the way to buy new games. Battlefront may look at the requirements from Valve (Valve's cut, sales etc) and balk, but they are missing the boat.