Ultimately the Pilates system of exercise is based around resistance. When doing Pilates on a mat or using Pilates equipment the amount of resistance and tension can often vary. This is due to either the body position, or use of small apparatus such as bands.
However, by using a reformer, cadilllac, weight stack, or other equipment this can be consistently regulated. Everything is designed to create planes of movement within which you can complete the exercises. The adjustable levels of resistance mean that you can progress your exercise in a very consistent way, ensuring a discipline of movement, and clearly demonstrable improvements.
There are over 500 documented exercises on the reformer alone, and with different variations of each of these in terms of position and resistance it's possible to tailor your exercise almost infinitely to create the right session for you.