I want to (a) be healthy and (b) look good. Being healthy is a long term goal which includes eating clean and being functionally fit so that I can enjoy life. Looking good is a byproduct of that, a really fantastic byproduct. I feel like when I look my best, I just have a better outlook about everything. Confidence is really important for personal happiness.
As for people having desires for very low BF, I think it's just one standardized scale that people use to compare themselves to others. I wonder how many people have a warped perception of this, because they actually don't know what their own body would look like if they were to reach such a low level (i.e. it might not be so hot).
Everything in moderation, including moderation.