Say you're developing a big form with lots of different inputs. When you're iterating on the form you don't want to lose all your form state because you edited some code.
When you make the change you want to see what that change does to your form in its current state.
That's very true. I think the most frustrating thing (that I regularly see) is people trying to do a minor fix in a wizard (let's say on the 5th step) and going through every step again just to see if the bug was fixed. Things like HMR can help here.
That's why things like mock-service-worker or having prefilled Redux stores (or whatever state management you use that allows something similar) are a real time saver.
Of course fixing the bug based on test is always better, but often you have to fix something visual which is a little bit harder to do with tests.
When you make the change you want to see what that change does to your form in its current state.