On can save time, money and frustration. Users only have one password, which reduces login time and frustration. Password resets are decreased due to the one-time authentication, which in return, reduces help desk support cost. Being able to do administration centrally improves the time it takes to add and remove accounts or modify access rights. SSO also improves security as users will no longer have to remember as many passwords and will be less inclined to keep a written list of them.
Administrators are able to confidently delete a user from all resources.
Backdoors are closed because a user only knows their SSO login, preventing them from logging directly on to the resource.