This registry dword, DeleteRoamingCache, controls whether or not, the local computer saves a copy of a user's roaming profile when users logoff.
Roaming profiles are stored on a server. However, by default, when a user with a roaming profile logsoff, the system saves an additional copy of their profile on the local hard drive. This scheme was designed to give roaming users faster logon, especially when network traffic was busy.
The incentive to change the default behaviour occurs when lots of roaming users logon to one 'kiosk' machine. As a result, the disk fills up with profiles, and if it's unlikely they will ever logon again, you may as well make a registry tweak which deletes these unwanted roaming profiles.