Joomla! multisites manager allows you to have multiply sites running in single hosting account and share database data between different Joomla! sites. Single login/logout on websites.
No hacks of core files are required and no Joomla! core files are patched.*
Ability to use database content, users (with Community Builder and JomSocial support) from other sites.
You can have many Joomla! websites using a single hosting account, with same users and JomSocial contents but with different template etc.
You can easily replicate your current website into other website without building new hosting account!