A downloaded part was drawn for someone else's thing: their fan, their drawer, their board. Yours is a little different: another size, another screw, another board revision. So the lid is tight, or the hole is in the wrong place.
The next file is someone else's sizes again. Every round costs filament, an evening, and a little of the fun.
CAD could fix that, but it was built for engineers.
You know the thing it has to fit. Loftsmith does the CAD. Your part is designed around your measurements, your printer and your material, and checked, before the first layer goes down.