Because you need to understand that a computer doesn't see a flashy GUI like we do. A computer only knows numbers 0 or 1, or basically on and off.
Dos teaches you to really understand the processes of what a computer might have to do compared to just simply clicking. I don't know why you would want to know DOS otherwise personally but to know the history it's good to know.
cls = clear screen
dir = directory
cd = change directory
cd..=go back a directory
ipconfig = shows i.p, subnet, gateway
ipconfig /all = well shows all
a command plus /? = shows you what all you can do with said command
that's really all you need to know, I can't think of anything else off the top of my head that I personally use whenever I bring up DOS. They might have you do something else though in class. It's been a while since I've had to do it.