In reply to keith-ratcliffe:
Pretty much what MrRiley said above.
I find it really useful if I'm hanging on a rope and the climber is coming near and past me relatively quickly. Because I generally shoot with an 11-16mm lens, I'm often within 1-2m of the climber, so large-scale recomposing of the shots is a given. This is exacerbated if I switch between landscape and portrait mode, causing the AF point I'm locking onto to constantly change. (I don't know why, but I always use a single AF point!) Having the ability to refocus or lock the AF with the back button,
independent of the shutter release, just seems to save me time and minimises lost shots if the climber is moving relatively quickly and I'm shooting & struggling to recompose and refocus.
It does take a bit of getting used to, but I doubt I'll go back to the 'normal' AF method of using the shutter release button. There are some very good YouTube tutorials on the subject, which convinced me it was worth trying.
Post edited at 17:45