Yes, cable tension keeps them in place, but in many or most cases, they're not necessary at all. The BB shell itself does the job, and the cable will find the shortest line and stay put without a guide. The guides main job is to keep the cables on a line symmetrical to the top tube for cosmetic reasons (so we don't get posts like "my RD wire is closer to the top tube than my FD wire, is this OK?). Also to prevent the wires from marring the paint under the BB shell.