That people are able to track their times more accurately.
Because what are they supposed to improve? Their memory? Make better estimates? Think harder? Forget less?
Keeping track of time is exhausting and distracting from the work at hand. The better you focus on your work, the less you think about noting times or punching stopwatches.
Anyone who actually believes that manual time tracking hardly costs any time - or even comes close to reflecting reality - is most likely not tracking times themselves.
Neither the time cost - nor the inaccuracy have to be.
If you have a memory aid.