CAD programs such as Fusion, AutoCAD, SolidWorks, OpenScad, etc all have their own file formats to store model information, e.g., .f3d, .dog, .scad, etc. many are proprietary but open. Thee are also platform independent standards such as STEP. CAD files can be used for many purposes, machining, injection molding, 3D printing, etc. Models are usually exported in file formats software used in the manufacturing process understands. For 3D printing, STL is the de facto standard. More recently 3MF has emerged as an improved format and is actively supported by Prusa. See -file-format-and-why-its-great_30986/. Could PrusaSlicer import STEP files and do the tesselation itself? Sure, but the line's got to be drawn somewhere. It's not unreasonable to draw that line at the export of a model from your CAD software of choice, with the specification of resolution of the exported model at that point.