Unit tests are theorems too! When you compute `Test(x)` and check it's `true` you have proven the theorem `Test(x) == true`. However that's only about a specific choice of `x`. Theorems allow you to use math to test all choices of `x` and cover infinitely many test cases.
In theory you could even write an unit test and offer a bounty to the first to write a function that's proven to pass it. So you wouldn't need to write it or review the results for correctness.
Can someone ELI5...
I quite like unit tests, but it's hard to connect proving bits of elementary maths like `1 != 0` to anything I do as a software developer. Not really clear why I should have to/want to.