Kids, here’s a real world example...
Say you bought your first house, hopefully not getting screwed because you don’t know math, but it’s a fixer upper with old wiring and a 60A electrical service. (There are still plenty of those in my neck of the woods).
You have some gift money from a parental unit to buy a spiffy new appliance that draws 1800W.
If you know math, and a little basic electrical knowledge, you can figure out whether or not you can plug that in safely where you want to plug it in without burning down your house.
1800W requires only 15A if 120v
Here’s my math on that. Get the electricians to borrow the panel and breakers from work, buy a case of beer, get electricians to install panel. Weld up pipe bumpers for electricians, drink beer and have bbq.
Total cost $22