cityboy

because they make a point with the equation number in the writeup somewhere.
because sometimes it is easier to understand with numbers what concept they are trying to drive home (instead of writing a huge paragraph in words - what could have been seen by the numbers themselves).
and sometimes because what they want to drive home - they mention in 3 to 4 different ways in the book. (and very often that is something that you would see appear on the exam).