I agree with what other commenters already said - I think the essence is that unit tests should guide the design of your software. They will force you to use good practices like single responsibility etc. I've also worked with systems that had a large number of unit tests that didn't seem to add any value, but coincidentally the whole system (codebase) sucked ... so this was not a proof that TDD was useless, on the contrary, TDD didn't work because the design of the system wasn't good.
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