Any contributions that you make - i.e. new lists, new list items and comments - will be identified with your chosen username.
Registration gives you access to 'My lists' where you can store your private lists and keep track of the lists you have made public.
You have access to your own RSS feed that keeps you informed if anyone responds to your contributions.
You can elect to have a cookie stored on your machine so you don't have to manually log in each time you visit the site.
You can specify a password so that you can log from any machine you want.
You can specify an email address so that you are informed if anyone responds to your lists, items or comments.
If you add a new list, list item or comment without first logging in then you will be automatically registered under the username 'Anonymous'. You can then take advantage of the facilities above, and give yourself a more memorable username should you wish.