Giving your factory a pattern without a tech pack and sample is like giving a kid a lego set without the instructions or the photo on the box.
They may know what the set is supposed to be, but they’re not exactly sure what pieces go where or what order to assemble them in. How do they know they even have all the pieces they need? They try it one way, but then need to take them apart and start over a different way before getting it right. If it is too complex a model, they might give up or switch to a more fun activity before it all comes together.
It is much more rewarding (and less costly on your part) to provide your factory with all the instructions they need to quickly and effectively put your garments together. Guesswork in production is no fun.