To be honest not using a calculator is so I don’t have to wait a minute for you to get out your phone, find the app, type It in, realize you typed it wrong, and then finally type it right just to do 4x5.
Like do you HAVE to be able to do this in your head? No, but it’ll make everything else we have to do a LOT less fucking annoying if you put in any effort because otherwise every step of everything you are going to do will be go to this thing and ask it what this is.
And most of math is mostly “can you learn something you might not enjoy and then use it to do other shit that takes longer than 10 seconds?” Most things aren’t particularly individually important. Idc if you remember the area of a triangle when you’re 45 but if you can’t learn something as easy as multiple the two numbers and divide by two and be able to use that then you’re gonna have bigger issues when you have to do literally anything.