The author writes

> I need to run tests, use goto definition and other editor navigation features, apply local changes to check if some things could have been written differently, look at the wider context to notice things that should have been changed, and in general notice anything that might be not quite right with the codebase, irrespective of the historical path to the current state of the code.

The editors/IDEs I've used usually only offer rudimentary support in diffs (goto defn, but only in the same file; maybe auto-completion, but usually not for newly added items; no refactoring functionality)