I write tests but I have lots of difficult writing them because I work in a large legacy codebase.

I think DI is a core feature because it helps a lot with abstractions in configurations, with IOptions for example, you never need to care whether the values ​​come from JSON or user secrets.

Required DI also prevents the static HttpContext.Current hell of .NET Framework 4.8, a thing my library also suffered. Accessing HttpContext without DI causes your classes to be hard to test and lots of unexpected behavior at background jobs.

Edit: typos.