Designs that are already print-ready, not files you have to fix
The first wall most people hit is not the press. It is the artwork. A design that looks fine on your phone turns into a blurry mess at full size, and you will not find out until it is already on a shirt.
Ready-to-press transfers skip that entirely. The file is already sized, already the right resolution, already printed. You are not designing, you are pressing.