[carry_bit]

Think of it as manual testing where your work is captured so it can be ran later in an automated fashion. There are many problems where verifying the answer is easier than coming up with the answer.

Asserting formatted output can also be really useful. A picture might be worth a thousand words, but when it comes to tests it can save you a thousand asserts. Writing those thousand asserts separately also would be so tedious that in practice you'd probably not write them all, leaving part of your output uncovered by tests.

When I wrote a LALR parser generator for fun, I added some code to print out a nicely formatted parsing table with debugging information. Besides being useful for debugging, it let me write simple yet powerful tests: I would feed the generator a grammar and then assert on the formatted parsing table. That made it easy to verify that I was asserting the right thing, and let me assert everything in one go.