Everyone's browsing habits are different, but I'll give you a scenario (i.e. the way I look around the forums): If someone has a particularly unusual / tricky question, and I
think I have some ideas about how to approach it, I will usually hold off for awhile. Better to wait for someone who knows what s/he is talking about rather than muddying up the thread with speculation.
But in some cases no one does reply, so it's nice to have a quick way to locate the thread again so that I can start hitting him with half-cocked ideas.
If no one cares, then that'll be that. I thought more people appreciated that feature.