Generally speaking the use case is writing tests. If your tests just call all the dependencies and new
directly then it’s harder to write tests for your specific component while avoiding setting up a whole bunch of stuff (to make all those other classes work). By requiring all the dependencies to be provided to the class, you can swap them out at test time for something else that’s easier to work with.
That said, IMO it’s a symptom of problems in language design. Using DI is only necessary because languages like C# don’t make it easy to mock out new
or classes used directly, so we resort to wrapping everything in interfaces and factories to avoid those features and replace them with ones that are easier to mock. If the language was designed such that those features were easy to replace during testing then DI probably wouldn’t be a thing.
Keep in mind it’s not an all or nothing thing, they’ll assign percentages of fault. It’s also important that they name name basically anybody involved because the others will try to blame Google to shift fault off of themselves.
Effectively you want to name everybody possible so that they all fight it out.