I'd like to begin with I do not have drive TAAATS but from what I recall of seeing it in action, there is a minimum amount of data that needs to be inputted for the system to accept a plan.
It can be done on the run, providing the controller can gather all the required details and guesstimate some of the others (I think a TAS or similar is one of the required fields). That probably takes 30 seconds to a minute to do assuming you aren't distracted by anything else.
Having a plan in the system, even if it does not have an accurate time in it, allows the controller to pull up your details simply by putting in the callsign. From there it can be manipulated a bit easier.
So a plan will save a bit of backwards and forwards ascertaining details to drive the system. It'd be similar to me suddenly clearing you for an IAL and you don't have your charts out. Sure you can do it, but you need a minute or 2 to get yourself sorted.