That certainly helps in eliminating some hard-to-control-for variables (like how much work time you will really have, as in what things will pop up that need ur

That certainly helps in eliminating some hard-to-control-for variables (like how much work time you will really have, as in what things will pop up that need urgent attention), and it also gives you "error bars" by default (4 days means 3-5 days), but lots of projects involve a bunch of unknowns, making them impossible to estimate with any sort of precision before you actually dive into them. As with anything, estimations should be iteratively improved, but usually nobody bothers ;)