For tutorials and decoupling loco dlcs from routes. They can't include all the trains due to lack of space and memory.
I understand the idea behind the training centre and I get that de coupling some dlc from routes is a good idea but I enjoyed doing route introductions and tutorials on the route they were supposed to be on. Just my two pence worth as I find the training centre a bit bland and a tiny bit laggy still. But as I said I get the idea for having it