Capt Stable. To take your example: We are West of the beacon heading North. We wish to intcp the 315R o/b. You can set the yellow arrow to 315 or to 135 and the BB will show the right picture. As you come onto the radial you know (because you know you want to go o/b) that you turn onto 315. I notice you refer to "the system" knowing which way to turn, and to the FMCS. Well that's different. Our aircraft mostly dont have those. They have pilots whose situational awareness is rather good.
(Edited to add: Thanks Bookworm, that's what I'm trying to say - and I needed some support!