Rational agent question from Russell and Norvig
This question is obvious as you told and its purpose is to ensure that the reader has understood a part of the chapter. 
You are right that the actions that the agent does will differ in the time period T. They will also differ after this period, because the agent's actions will have no value.

One example may be for a car agent that has to cover as much distance as possible and has limited fuel. If only the distance reached during the first hour counts, the car could take it in full throttle(exhausting the fuel very fast). However if there was no time limit the car agent should choose the optimal speed that maximizes the distance covered per fuel unit spent.