One of the password flow use case is; there are legacy clients that can not support redirections; but you still want to be sure that your credentials are handled central (your Identity Provider). Even it is not suggested flow to use, still could be valid reasons to use. In this case implicit flow or PKCE can not be used. You can imagine your backend still get benefits of not storing user credentials. Also your company manages all users central, more secure then exposing every credentials to different services (logs, their db etc.).

One of the password flow use cases is:

• there are legacy clients that can not support redirections
• but you still want to be sure that your credentials are handled centrally (by your Identity Provider).

Even if it is not a suggested flow to use, there could still be valid reasons to use it. 

In this case, implicit flow or PKCE cannot be used. You can imagine your backend still gets the benefit of not storing user credentials. Also, your company manages all users centrally, which is more secure then exposing credentials to different services (logs, their db etc.).