For example if the magnification on the scope says 1000x but is actually 1050x, the item you manufacture and measure would have wrong dimensions that might cause issues during its use. This is why you calibrate them, so you know that something that's meant to be 1000 micrometer is actually 1000 (+ or - a certain tolerance that it's certified to). This is also needed for the certification of the products you manufacture.

However if you're just a hobbyist and want pretty pictures or you don't care too much about the accuracy, then it's not necessary.

I worked in a place with an electronics test lab and even stuff like multimeters would need to be certified yearly. This cost a lot (a significant fraction of the purchase price!) so often it would only need to be done for a few years before the meters would get replaced entirely. They'd be useless for the lab then, however they were still fully functional for home use as my need for accuracy isn't that high. Normally I use cheap chinese meters but having a real quality fluke is a big step up even if it's not calibrated officially. Unfortunately most of the stuff was scooped up by people a bit higher in the food chain than me :)