If you mean, "why can you ask a question" then that is an option in Q2A. Here on the Question2Answer Q&A you don't need an account to post (but all anonymous posts must enter a CAPTCHA I think). But on my site for example, you must have an account, and you must have your email confirmed.
There are quitefine-grained controls over permissions in the options, check out the demo for more.
