The interface enables users to easily add, change, and remove application specific variables and paths from their environment. An environment manager becomes useful at large sites where the administrator must support many software packages on many different platforms.
Users seeking a consistent environment usually end up writing fairly complex shell startup files.
These startup files tend to be filled with nested if-statements, multiple hostname and uname calls, and a myriad of other bits of logic. We want a user to be able to login to any client workstation, regardless of the platform, and have it behave the same. We don't even want them to know where software is installed. We don't want to have to tell everyone where the new version is. It should be transparent to the user.