There is a nice login facility available in joomla websites. Let me know for what purpose do you use this feature in your joomla websites?
Basically to allow user login. It can be for ecommerce, comments, discussion, sharing, or even social network (joomlsocail or community builder).
Sarah has already covered all of the things I use it for. 1. Login for jomsocial, which I call the Fan Zone on my site (sports oriented site) 2. Login required for commenting. This cuts back on the spam a bit. 3. Login for authors to add content to the site from the front end Lots of other reasons for why you would use the login. These are just what I use it for. I have some people who tell me to disable it entirely though because they do not need (or want) people to be able to log in.
It has same features as that of Wordpress Admin login and gives functionality to control the Joomla website from back-end. And you can add more features, Ads, Revenue making methods, Traffic Tracking, etc to the website.