Why are circles used to draw a logo?

Recently, I started taking interest in logo designs and started replicating some famous logos.

Soon, I came across this article about twitter's new logo, released somewhere in june '12 . On this post, I learned the fact that circles and the mathematical constant golden ratio are being used in logo designs of twitter and apple and icolud.

I want to know, what's the reason behind this? Is it a matter of good practice or is it some standard practice?