Type systems and unit tests have something in common: they are both in the category of 'documentation', alongside comments.

They differ from coments in that they are designed to be machine-interpretable, but like comments they do not modify the behavior of the code if stripped out. This is the key principle of documentation: it's additional information in a program that does not modify the behavior of the program when it is stripped out.

They are vital for ensuring the correctness of the program. But 'correctness' is a human concept; programs do not care.