There are several reasons, actually.
One of our main goals as a forum is to attract new turkers so we can continue to grow. New turkers often aren't all that confident with finding their own HITs, so they tend to go where they can find HITs that have been vetted by others. It doesn't necessarily mean that they need to be given a completion time for everything, but just being told that "hey, this looks good" can be a big help.
Also, having a lot of HITs posted in the daily thread tends to keep us on topic. When people stop posting HITs, or aren't posting as many, they tend to start talking about other things. That's fine for a bit, but it can often go on for very long periods of time. That looks terrible to anyone that may be trying to decide whether or not they'd like to join the forum. We often start spamming HITs to try to get people back on topic when there's been a major thread derailment and it works. It gets people talking about work, again.
Another thing is that it helps to advertise our forum. We've had new members join in the past that said they just came here because they saw that most of the HITs that were showing up on HIT Grabber had been coming from here.
Of course, all of the reasons that I listed have to do with bringing in people that are newer turkers. More experienced people may look at the HIT spam in the threads and consider it unnecessary, but there are plenty of people that do appreciate it. And, in time, we hope that those people will become more comfortable with finding their own HITs and start contributing some of their own. It's just a great way for us to continue to grow as a forum, I think. It also cuts down on the "this forum sucks, where are the HITs?" posts that tend to pop up on reddit from time to time.