What is the purpose of User Groups in AX 2012?

Dear Team,

Kindly let me know that what is the actual purpose of User groups available in AX2012.

I want to know particularly that how can I relate this with security roles. Will user groups come into the picture of security roles in any of the case. Can anyone give me some examples?

In AX 2009 User groups played an important role in providing permissions, but in AX2012 we have Roles, then what is the purpose of User groups in AX2012.How it is related to Security?

Regards,

Divya Lakshmi.J