When they are for software as a service arrangements they are sometimes known as click-wrap agreements, due to the pre-history that exists for shrink-wrap agreements. When software was commonly sold in boxes, a written agreement came in the box and the practice developed that breaking the shrink wrap was treated as acceptance of the licensing terms and conditions.
Preparing the terms helps think through, conceptualise and document the operation of the site, area by area, section by section or even page by page.
This helps build practical risk minimisation customised around the information architecture or process governing or controlling the user experience.
Contract formation (if appropriate) between the operator and the user
Intellectual property ownership, control and amendment of content - all of which is important for both website and user generated content
Liability limitation and exclusion for the operator
Warranties and indemnity, if any, given by the operator
Acceptable use rules to apply to all user generated content
Account suspension, termination and consequences of termination
Governing law and other general or "boiler plate" clauses
Legal risk to the operator of a website can also be further minimised with other measures and mechanisms, eg appropriate business and enterprise structuring, but those steps are beyond the scope of this short article.