Is that useful if you’re never writing django apps?

It depends on your background. Having written web app before let's you quick grasp the ideas laid in the book.

To me, the most important chapters are

- https://www.obeythetestinggoat.com/book/chapter_mocking.html - https://www.obeythetestinggoat.com/book/chapter_purist_unit_...

Having said that, the concepts are universal.