This is done to prevent spamming. We have had a lot of spammers (this is a very common problem, as you might imagine). Some spammers post commercial ads right away, while others will post utterly useless filler, as an excuse to add a link about their own websites. For example, a spammer might post something like this:
Great post! I am really looking forward to more of your insights on this subject.
Notice the total lack of any original content, or the fact that this post could be added to any thread on any subject with no modification. The point is just to get their sig link or user profile link on your site. There are even "bots", ie- automated systems which spam these kinds of useless complimentary posts all over Internet blogs and forums. Some of the more clever bots will actually try to appear relevant to the subject by examining keywords, although you can still tell it's an automated system rather than a human. Therefore, keep that in mind: empty-headed posts like this set off our "spammer" alarms; it is best to say something that indicates that you actually read and understood what people are saying, so that you are identified as a human rather than a bot.
Once you've had a few posts approved as being non-spam, you get moved into the regular user population, and your posts will show up without requiring moderator approval.