It really depends on the airline. Many airlines don't have them any more. Traditionally, they would help with dispatch duties but not actually release flights (Jepp chart revisions, etc.) There is no hard and fast answer as to what the position entails since each carrier does things differently. Usually it is a stepping stone into regular dispatch. The only airline I am aware of that starts everyone out as an assistant is Southwest. I think UPS may do so also but I'm not sure. At my airline, the assistant desk does a lot of calling people for overtime, setting up charters, and listing FAA inspectors for jump seating.