Perhaps you have some history with them, and something current reminds you of a past conversation, so you want to resurface it.
Perhaps you've realized that they are well-educated in a field, and have a few questions to ask them but want to keep the conversation light-hearted and casual.
Perhaps there is a thread that you really want them to see.
Perhaps you like their avatar, user name, sig, etc.
There are a whole slew of reasons why we might want to contact somebody, outside of responding to their insight given by a specific thread/post.