Why use percentage value for div's width in CSS?

Question

I am reading articles about css. I found out that many of writers suggest to use % value for div\'s width or height.

I am using pixels all the time.

Why should I

Answer 1

I find web sites that don't fit the screen, either leaving white space (which can be considerable) round the edge of the content, or falling off the side with a scrollbar (or, worse, falling off the side withOUT a scrollber!) intensely annoying.

I write my pages to fit the screen, regardless of how large that screen is.