Why is "shrinkage" taken into consideration when calculating water volume?

Since I'm starting out with a measured amount of cold water that gets heated to strike/sparge temperature and ending with a measured amount of chilled wort in my fermentor, why should I care about shrinkage? It seems to me that I would only be concerned about shrinkage if at some point I was measuring the volume of my wort when it was hot. When is it necessary to do that - I only care about the amount of cold water I need to heat for mashing and sparging. What am I missing?

If you have a pot that lets you know how much volume you have in it - say from a sight glass, then while you're boiling off you can see if you've hit your target volume. If you don't factor in that hot liquid takes up more space than cool liquid, your target volume shrinks while cooling and you end up with less beer than you originally thought.

If you don't have any volume measurement on your pot, and you're just assuming a specific boil off rate and length of time, it really doesn't matter.

some people monitor their boil kettle volume so that they know when they hit their target. Evaporation rate is not always constant, it can be affected by ambient temperature, humidity, barometric pressure, wind(i'm kinda guessing on this) , etc... It's just another way to control repeatability.

When I'm doing a really big beer, I have a longer boil, so more sparge water, and a larger preboil volume. I monitor the boil for the first hour or so until I get to my 1 hour volume so that I know when to start my hop schedule.