Let's say I see a long thread - 20+ posts about some bug, crash or other problem. Sure I can read the whole thread. I suppose if you are in thread view you can just do a ctrl+F in your browser. But you know how in Google you can search within a site if you know the site. Imagine if you could click "search in thread" and do the same.
Here's an example.
Let's say the initial post in a thread is "Import Process Server error". You check out the first post, read the question, then start reading the thread. But sometimes others chime in to rant about the problem, before Tim Kolb chimes in at message 25 with an answer. In the forum all messages display, perhaps I could mouseover the initial topic, then look for the likely answer without clicking the thread or the various posts hoping to find something useful.
I know, I know, more clicks on the site are better for the site. Just thinking out loud - maybe not so logical.