If you don’t diff before you commit you don’t know what you’re committing.
When programming anything even remotely interesting, we’re making changes all the time. Change, change, change. Phase 1, phase 2, experiment 1, experiment 2, etc. And we’re using Version Control Systems, like git. (If not, stop reading and learn git)
If you believe as I do, you should only commit working code, then we must safeguard our git repos from danger from ourselves. That’s why you should always run git diff and scrutinize the changes before git commit, to make sure debug, merge conflicts, and other unwanted changes are not making into the big time.
It’s like look both ways before you cross. Diff before you commit – make it a part of your stitch in time.