Hypothetical situation: You are attending MVMCP or MNSSHP at MK later that night, but want to tour Epcot for the day. Park at TTC, monorail to Epcot, come back to MK for party and your car is in the TTC lot.
Or, maybe MK is open late and you plan to finish your night there, so you park at TTC, monorail to Epcot for the day. Maybe you have a dining reservation, or you have a few drinks but no worries because you don't have to drive back to MK - just monorail. By park close, you've sobered up, your car is in the TTC waiting for you and you go home.
Just off the top of my head, these two came to mind. I'm sure there's other reasons people might park at TTC but include an Epcot stop in their day. *shrugs*