You don't need to, but (Dokku, at least) has a lot of ingress setup and Procfiles and things that you need to add, and you pay that cost for nothing if you don't use it.
The other thing that makes Dokku unsuitable for non-web things for me is that it's explicitly geared towards the web setup, with ingress/web server/database, and you can't easily add arbitrary services and link them to your container. I remember having to do quite a bit of hackery to deploy one server for the backend and one for the frontend on the same domain.
