Single sign-on (SSO) allows a user to authenticate to one system and leverage this to use a different system. There are many different technologies that can be used to connect the systems for authentication purposes. These include SAML, OAuth, or OpenID.
Single sign-on gives more control back to the organization by allowing them to control the accounts. This control can include who has access, password or other authentication policies, and ensure that access is removed if the employee leaves the organization. SSO also reduces the number of passwords that users have to remember, and decreases the chances that the additional passwords are stored insecurely or that the same password is reused where it may be compromised when used in the external system.
Organizations want to be able to authenticate against their internal authentication system. They want a standards based authentication mechanism which may include SAML, OpenID, OAuth, OAuth2, or other such open mechanisms. Companies want to be assured that the method is secure and that it does not require development and support on their end. There will always be an element of support, but it is minimized by using standard authentication mechanisms.