What the STEP file does well
A STEP (or STP) file gives the shop solid geometry it can open without your CAD software. It is usually the starting point for programming, picking stock and planning the job.
For a simple prototype, the model plus a short list of written notes is sometimes enough to quote and make the part.
What the drawing adds
A drawing can call out the things a model does not show on its own.
- Tolerances and which surfaces they are measured from
- Threads and hole callouts
- Surface finish and edge breaks
- Material, heat treatment and coating
- Part marking and inspection requirements
- Revision level
When you need both
For a part you will order again, send the model and the drawing together. That gives the shop both the shape and the rules for accepting the part.
Make sure they agree and show the same revision. If the drawing overrides a model dimension on purpose, say so on the drawing.
A perfect file can't answer an open question
If the material, finish, tolerance, inspection or quantity is still undecided, say it is undecided. Don't leave the shop to guess.
That is when an early review helps more than asking several shops to re-quote every time something changes.
No CAD file at all?
You can still start. A sketch, photos or a sample part are enough for a first conversation, and the model or drawing can come later.