When would you test an unknown bacteria for flagella?

I know how to do it, I just don't know when I would want to.

When would you test an unknown bacteria for flagella?
Its one of the tests you can use to distinguish between unknown species (i.e. if the are motile they have flagella). If your other tests (gram staining, etc) fail to distinguish between two species, this is just one more test you can try.