It's the main thing because colesterol is the fat in your blood and if you have too much of the wrong one you clog your blood vessels and will die (sooner or later).
There are also other measuring units for fat/weight/health like BMI (body mass index) or your waste diameter/circumference or thickness of your skin aso. But colesterol gives you a more direct hint to the state you're in.
I just saw a TV show on Discovery Channel about the book "You - a user's manual". Sounds very interesting. Written by two US GPs, not extremist, and very practical, me thinks.
Dani