For one thing, just displaying the latest number isn’t useful if you’re doing anything complicated. For another, many calculations involve using the same number over again multiple times. Some calculators have a memory entry, but many don’t. There’s a “C/CE” but there isn’t a backspace, so if you get one digit wrong, you have to start that entry over (and hope you chose the right option among C/CE/AC/CA/etc. If you accidentally hit the wrong operation key (multiply, divide, plus, minus) AFAIK there’s no way to clear the operation. A lot of common math operations involves parenthesized expressions, but if you’re using a basic calculator you have to instead enter things in an unnatural order. It’s pretty common to end up in a situation where the calculator is displaying B and you want to do A/B but you can only easily do B/A. Fancy calculators have a 1/X button to fix this, but if not you’re out of luck. Same with having B and wanting to do A-B but only being able to do B-A. You can fix that by multiplying by -1, but again, it’s a UI issue that you can’t just say “hold onto that number for a second because I want to enter another number and then use it”.