User logins in dotCMS come in two flavors: frontend and backend.  The main difference is that when a user logs in to the front end, this is very much like a traditional account login system you use at many other sites.  It enables them to take advantage of permissioned content you may be using, or allow access to special sections of your public website.  This also allows you to do tracking of specific logged in user's browsing habits within your site(s).

Backend users, however, are should be extended role permissions which give them access to the tools that build your site (as their permissions allow).  You should keep these two scenarios in mind when creating user roles, in the event you need to provide separate types of user access to different parts of your site.
Additionally, users in your system can be connected to a host of other features, such as user tagging, tracking, mailing lists, keep notes on users, etc.  You can easily integrate dotCMS with your current user databases as well, such as LDAP, to quickly merge systems together.