Sometimes an STL file needs a bit of editing, but if one doesn’t already know how to use a mesh editor it’s unlikely that one ...
You have finally set up your 3D printer and found something interesting to print. The model looks  perfect in the download ...
If you think about it, STL files are like PDF files. You usually create them using some other program, export them, and then expect them to print. But you rarely do serious editing on a PDF or an STL.