The reasoning is probably, so you don't sign up in the forums, make your post to get verified, then get verified and then never visit the forums again. So it's an urge to get you connected to the community.
I'm pretty sure that's it, so that you post some stuff, that leads to posting other stuff, which eventually get's you integrated into the community. The difficult thing is to come up with stuff to post in the first place
I think it serves a valid purpose. Before, you would just get people that signed up to become a rep and never looked back here again. Now, when people already showed some interest, they are more likely to come back and check on what's going on.
I think it's just to make sure that the verified members are people that are actually willing to show some involvement. Ofcourse, you might get ''pointless'' posts, but they've at least taken a look at what this forum has to offer. That's my opinion, anyways.
We are monitoring the introduction and general sections a bit more now, just to look out for people who are trying to cheat the system. The rule exists because of exactly what Raptor and Raymen both said. One great example was when someone did a 5-word intro and separated all 5 words into 5 different posts.
People should also bear in mind that mods can alter people's post counts, so we could in future just reset people's post counts to 1 if they are abusing the system and then send them a message telling them not to do it like that..