1) You send an email bcc'ing a bunch of people. Those people start responding, Gmail threads it into 1 thread. Sometimes you want this and sometimes you don't
2) The topic of a thread changes and you want to split the thread into two logical threads.
Currently we have this limitation where the new thread has to have a different subject but we're working on removing that restriction. Should be a few more days before thats ready.
1) You send an email bcc'ing a bunch of people. Those people start responding, Gmail threads it into 1 thread. Sometimes you want this and sometimes you don't
2) The topic of a thread changes and you want to split the thread into two logical threads.