There is sometimes a good reason for putting links to old threads in a new thread. Sometimes it's useful to link related threads together for anyone who comes along later and wants to check them both (all) out. It's much easier to keep track of discussion on a particular subject if the threads on that subject are linked.

I'm sure there are cases where this wouldn't apply, and it wouldn't be appropriated to link an old thread to a new one, but it can sometimes be useful.