A User Account allows you to -
- have "breadcrumbs". That is, you get a little navigation aid that lets you see the previous pages you've viewed - handy.
- be notified by email if a page changes, allowing you to monitor progress. You can monitor individual pages, or changes to any page on the site. To enable global monitoring of pages in your login preferences, go to the "Subscribed wiki pages" and add a dot-asterisk. Like this -
- set the look and feel of the site for your personal taste.
- various other things - have a look.