This stops "fly-by-night" users (bored kids, silly users, spammers) create and account, flood the site with lots of questions and leave. The limits encourage serious users to use the resources wisely and earn points to move them up levels as well as gain experience using the site, get to know people, and establish their reputation and user network.
Also... by having limits, it is easier to "weed" out trouble makers at the early stages.
Hope this answered your question.
Cheers from Australia