It makes sense, but common core can fuck off. I despise over-complication in all aspects of my life. For math, I always hated showing my work because I could easily and quickly do math problems in my head. I was the only kid in my class that would do math assignments in class without a calculator, and I was always the first one to turn in my papers. I'd still show my work when necessary, but it was just a waste of time for me. If I had to do common core, I probably would've dropped out, no joke. Fuck drawing diagrams to subtract two numbers. These kids are going to be lost in the real world. They're going to get paralysis by over analysis. They'll get a thousand yard stare on their face if you ask them to give you change for a 5 dollar bill because they'll be trying to visualize a damn diagram to figure it out.
You say it's ridiculous for kids to learn subtraction with diagrams they need pencil and paper for, but this:
is exactly that. It's a diagram. You can't easily do addition, subtraction, or multiplication this way in your head, lining up columns, subtracting them one by one and carrying tens digits. You'd quickly get lost. It only works with a pencil and paper, which is exactly why so many of today's adults are already "lost in the real world", as you say.
You were the type of kid who could do problems in his head, without a calculator and without using the methods they were teaching you. So was I! I feel your pain. That's because you figured out your own way for working out the math that didn't require writing it down.
And that's what this Common Core math is trying to teach all kids: the very same methods that you and I figured out on our own. That's the whole point: unlike the old methods for doing math, once mastered, these new ones don't require a kid to have a pencil and paper. Because they're the same methods you and I used in our heads.
The diagrams are only there to help the kids who aren't as instinctively good with numbers as you and I were: to help them visualize the in-your-head method for doing math and show they understand it. Unlike the one in the picture above, these diagrams aren't designed to be leaned on for life; they're training wheels.