Enabling session.auto_start vs session_start();?

I have created a test in my site to ensure that everyone enters only after clicking a box on the front page. This requires an if{}; check at the top of each page loaded to determine if redirection is needed.

It is working just fine, but I wondered if there is a reason I should NOT just enable session.auto_start instead of calling start_session(); at the top of every page of code?

Thank you