STL is most strongly associated with
3D printing, additive manufacturing and rapid prototyping, where the downstream system mainly needs a printable surface rather than full CAD intelligence. It is also used for scanned meshes, concept models and manufacturing preparation, and as an output format for tools that generate meshes from precise CAD geometry.
For software developers, STL matters when building applications for mesh import, model repair, print preparation or conversion between mesh and CAD environments: a slicer that cuts STL files into layers, for instance, or a scan-data pipeline that cleans up and re-exports meshes.