Informing instruction
Even with carefully written distractors, as in the "night and day" example, it is often hard to know why a student got a question wrong or right. But unless a teacher has that information, the test result is not useful for improving instruction for the individual.
A standardized multiple-choice test may point to some broad areas that need improvement. For example, a test may show that students in a school or district need to improve on double-digit multiplication. However, the tests do not provide information that will help teachers do a better job of teaching double-digit multiplication because they do not show why the class generally did not do well.
If students were asked to explain how they got their answers, then their teachers would have a lot more information. This information is vital for teachers to make instruction more effective. For example, students who did not know why "the earth spins on its axis" is the correct answer to "night and day" but happened to guess the correct answer would be unable to explain why. Their mistaken views would be visible to the teacher, who could then address the misunderstanding and clarify the concept.