I remember when I was taught, "comments are the most important part of code." Or rather, when I was told that comments are the most important part of the code. I don't think I was convinced, and I still see common cases where programmers are not convinced of the necessity of good & thorough comments.

I am certainly convinced myself at this point - trying to read, in particular, complex formulae that call functions that call other functions that I don't understand - but I don't know how to convey this to students.