If you think about it...all towers (and radar facilities) are not the same. The ATC procedures (the way we talk to aircraft) are the same but not the airspace, runways, taxiways, agreements with adjacent facilities, etc. All controllers go through a basic ATC school where they are taught..well, the basics of air traffic. When they arrive at their new facility they have to learn everything about that facility and procedures. Until they satisfy the requirements for a rating in that facility, they are in training. I have been rated at 7 different towers and was a trainee at each one until I was rated. Some can take a day for an experienced controller, others can take up to a year; it depends upon the complexity of the facility.