It's just how I believe that tests should be named. It's how I've always done it in the past. It's better, imo, to say that we are testing _that_ something works, as opposed to just say we're testing something. `test_that_a_new_user_can_sign_in` exactly describes the test case, while `test_sign_in_for_new_user` could be interpreted in many ways. Are we going to test what happens when a user enters invalid credentials? That could be included in `test_sign_in_for_new_user` but not really in `test_that_a_new_user_can_sign_in`. When other people are looking at test results it is nice for them to be able to have a good idea of exactly what the test was testing, without having to look at the code.

Is that reason enough?