There are use cases where a web app is not suitable. For example:

- Sensitive data that you do not want to be known by thid parties

- Very interactive applications where consistent subsecond response times are needed

- Applications that rely on data located in the computer (for example from other desktop applications)

Are you asking why a desktop app in general or why an electron based desktop app instead of a web app? I think it is interesting that the other two answers (at the time I am writing this) answer each potential interpretation of the question.

Exactly. I can't think of any use case where I would want to use a desktop app that doesn't have the native performance of a real native solution instead of just using a web app.