I get asked this a lot, I don't personally view it as excessive, but the reason I do it is when I'm bored with the current threads. The rules only say to avoid bumping old/inactive threads if the topic isn't relevant, so I only post in threads where the topic feels timeless. If there is already a "do you prefer coffee or tea?" or "what pets do you own?" thread, I don't feel those topics are time-sensitive enough to warrant completely re-making them. I never post in inactive threads related to completely irrelevant topics, like very old political threads, or threads about movies that came out years ago.