The most useful thing we ask an engineer in an interview isn't to build something. It's to read a piece of code they've never seen and tell us what they would delete.
Writing from scratch is the easy half of the job. Most of the work on a live system is arriving in the middle of something someone else built, under deadline, and figuring out what is safe to touch. Deletion is the sharpest version of that question: to remove something confidently you have to know what depends on it, what happens to the data when it's gone, and what nobody wrote down. Strong candidates slow down, ask what runs in the background, and name the one thing they wouldn't touch without checking first. Weaker ones tidy up whatever looks ugly.
We like it because it can't be rehearsed, it takes about twenty minutes, and it predicts the thing clients actually feel: whether a new person can join a running project without breaking it.
What's the last thing you removed from a working system that turned out to be holding something up?