The <doctype> tag isn't absolutely essential. It's all about standards in HTML.

The purpose is to let a browser know which version of HTML is being used and, subsequently, which tags to expect. Should a browser come across an unknown tag then it will be ignored.

http://www.w3.org/TR/html401/struct/global.html gives more information on doctype and how it can be used.

I was reading that too and noticed alot of the big sites don't use them either.

Geeks trying to complicate things i bet. lol

Are these important, i've noticed alot of the big sites don't use them but do they have any effect on quality or compatability in browsers?

Cheers.