Well, it's free for one thing, but there are better reasons as well. Registering allows you to join in the discussions as well as post pictures in them, there may also be content that is unavailable to unregistered guests.
If you are a beginner, there is a place for you to post those first photos and get good feedback on how you're doing, you can also ask questions on how to get the best out of your new camera.
If you are more advanced and want to get even better, you can upload pictures to the Critique forum for honest feedback.
Finally, and possibly the best reason for registering is that it is an opportunity to join and contribute to a friendly community whose members have a passion for photography and making pictures in common.
Registration is free and takes a couple of minutes so please, come and join in.