It's an attempt to prevent the internet from having multiple standards with each browser doing what it wants.
Without such an attempt, you could end up having to install diferent browsers just to see diferent pages correctly.
Hey... actually you already need to do that.

Some pages do require you to use IE to use them (*cough*WindowsUpdate*cough*), some don't work properly in other browsers than IE, and some correctly written pages fail to show properly in IE because it needs the page to be written in malformed language...
As to why it's bad that some browsers aren't standards-compliant... Honestly, it's like if someone went through a fast course of Sindarin, Klingon, Huttese or other made-up language, then started speaking badly butchered version of it in some Nerd Convention of your choice and claimed it was completely proper and everyone should use that version of the language since that's the best that this person can do. Microsoft didn't invent html, so they shouldn't be the ones to try and dictate how html should be written and interpreted.
Oh, and I'm using Opera. Mainly because it has innate mouse gestures and I'm used to the configuration, whereas FF needs you to find and install the mouse gesture addon of your choice, and then configure it to your liking. Which isn't a problem in itself, I just can't bother.

If I needed extendability, I would use FF.