They can be used in place of a shoulder plane if you don't have one. I don't. Most bull nose planes have a removable nose piece that allows them to act as chisel planes as well. Cleaning up stopped areas, trimming tenons to fit, etc. The downside is the short nose area. For things like shaving down proud dovetail 'fingers' there is not much to rest on the case as a guide. As I can't afford both, I believe I will go with the Veritas bull nose instead of a shoulder plane AND a chisel plane.
Any sufficiently advanced technology is indistinguishable from magic.
- Arthur C. Clarke