For me, a distinct advantage of on-line check in is to avoid that BA
strapline "Sorry Sir, our Club World cabin is overbooked this morning. As you
were last to check in this morning, you've been downgraded. Have a nice day -
all 13 hours of it stuck up the back."
Haven't used them since - till this coming November for unavoidable reasons. We'll be checking in on-line.
It can work the other way. I recently received unsolicited upgrades from WT+ to Club World on both outward and inward legs of the same trip!