I’ve never really found anything unique enough that I’ve felt the need to purchase it over a free option that is available. Frankly, these days I tend to get frustrated by all the obviously bad models out there and just use my CAD skill to properly design exactly what I want.

Also I’d never purchase just a STL file, I’m opposed to the format because it is so difficult to modify. I might consider buying a STEP file if it does something unique and useful to me while saving a bunch of modeling work.