The TTC is used for:
-Transferring from MK to EPCOT (via monorail).
-Tranferring from MK to MGM or AK (via bus)
-Transferring from MK to Good Neighbor hotels (via scheduled non-Disney buses)
-Transferring from MK to non-Disney resort (via scheduled non-Disney buses)
-Transferring from MK parking lot to MK via monorail or ferry
-Transferring from monorail resort line (Contemporary, Polynesian, and Grand Floridian) to either EPCOT line or MK Express line.
-Going to MK via monorail or ferry after walking from SoG and vice versa (or riding SoG bus)
-And purchasing tickets (you can also do this at any theme park front gate).
This is all I can think of.
Originally, they were going to have one or even a few TTCs as transportation hubs for all WDW properties, but they are moving away from this.