I can think of several other reasons: - A/B testing. - Limiting the number of articles a non-paying user can read per month. - Persisting form data and shopp

I can think of several other reasons: - A/B testing. - Limiting the number of articles a non-paying user can read per month. - Persisting form data and shopping cart info. (Not all sites require an account to order stuff from them.) - Improving recommendations based on what someone has liked or viewed on the site.