- Drill out sheared shafts.
- Drill a shaft to use a T-piece.
- Drill out a damaged thread.
- Cheap ones will bend or fracture.
- Buy titanium nitride tin bits if you can afford it.
- Can be sharpened to extend usable life.
These are the cheapest bits suitable for drilling metals and usually come with a black coating to help prevent corrosion (when undamaged) and slightly extend life. There are several advanced coatings which extend the life of this type of bit and might help to prevent damage if you are a bit lax about cooling and lubrication.