If I remember correctly, it scans system files and replaces broken/corrupted ones. It can work on some issues, but it’s not a fix all thing.

It should fix system files that are not in expected state (I assume corruption, missing, wrong permissions etc.). Maybe it was more useful in the past, but after trying it couple times around 8 years ago and never seeing any benefit, I have never thought of using it since.

My colleague said it fixed some random issue once or twice after he was out of ideas.

If system is truly messed up, it’s often faster and more reliable to just reinstall it, especially if you do not have much custom config.