How to use focus points

[Toby]

I'm a bit confoozled here.

I generally set my cam to use the centre focus point only rather than all of them. What I can't wrap my head around is how exactly things work with multiple focus points.

If I use them then, when I focus, some of them will get a "lock" and flash red but what exactly does this mean? The camera can only focus at one distance so is it just pot luck which focus points it chooses?

I can see how multiple points could be useful when using the A-DEP setting (which I don't) to allow the camera to work out the aperture required to keep all the focus points within the DoF but, in other modes, how does it work or help?

Any advice appreciated, sorry for the noobish question