Who would want to buy stock prescription lenses

[cijii]

Hi, I'm new to the whole industry and still studying the structure of the supply chain. One thing I'm not sure is which part of the supply chain would want to order stock prescription lenses (in bulk or small amount)? Is it the optometrists, opticians, and ophthalmologists who buy them and let the lab to further cut and grind them? Or is it the labs​ themselves order the lenses and finish processing them and wait for the eye professional to buy? Or is it someone else?