Train might stop in places that aren't stations. Also different stations need the doors on different sides to open. Therefore its useful to know exact location of train.
GPS could still be used for that, but what is wrong with opening the doors in the tunnel not at a station? I would think that would be helpful in the case of emergencies where people need to get off the train between stations. If a driver accidentally pressed the wrong button they might want to check to make sure no one is outside the train when they drive off, but I would think that wouldn't happen to much.