The idea is to compensate for people who glaze over those points.
Most often, though, people just forget, or have other commitments at the time.
I might be interpreting things differently than you but I do not see where it says you have to login back to check on new messages posted on your Introduction thread. But as Verkseid said, if the system pokes inactive people then that should be enough of a reminder.
You'll often see a spike in approvals during the first week of the month because of it. Rather few people are actually confused, though I have gotten a lot of thank yous. It happens, though, and part of being a human driven community is being able to provide individual assistance to people : )