Why do all math teachers make us do so much work?

when will we ever need " x+y=z " in our life?

We use math everyday in our life, without even knowing it, if you are to pursuit a career in engineering math is the basic concept of that career, you also will use if you ever are a cashier, there are endless careers that involve math

It's not always because we will use it later on. Sometimes it is to stretch, challenge and train our brains.