Good question. I've often wondered that myself. I sorta knew 142 was for sim schools, but had not looked closely at the 'Applicability' part of the reg, and some of the details, so I did that.
Looks to me that it is applicable to all parts of training. The difference is in the way simulator training is allowed. Sim instructors don't have to be FAA instructors. All training conducted in aircraft must be with FAA CFI's, so it is written for sim schools which conduct type training with experienced type rated pilots who are not CFI's. But you could do other pilot training in aircraft under the 142 certificate meeting all the 141 or 61 instructor/syllabus/aircraft requirements.