To some extent, each thread is a "conversation", so even after a specific answer is given or the OP's trip completed, the conversation can go on.
My guess is many folks come here (for the first time at least) with a specific question in mind - like "nice hotel in Paris" - and, if they're like me, run search or scroll the France page for an answer before posting. In that case, they may join an older thread (possibly ignorant of the age of the thread as a newbie here), and that often gets folks to revisit the topic again - maybe with new eyes.
I like seeing an older thread that I've forgotten (but responded to) where my immediate answer to the OP is pretty much the same (or way different even). As I read through the thread and see my original response I get a chuckle at my "past self" and my lousy memory that made me forget I had replied already.
I actually struggle with when to answer a NEW post with the same question again - eg. the "nice hotel in Paris" or "restaurants in Amsterdam" or the like. Is it worth rehashing a reply that already exists in a different but searchable thread?