So that you are aware of which functions you use which may fall over in IE and how directives should be rendered.
And, I guess, a good set of backup info if ever you get into one of those arguments about which browser should rule the world. (Although it's far better to never ever lower yourself into having such a debate)
http://www.howtocreate.co.uk/wrongWithIE/
Also quite an entertaining read in places, complete with conspiracy theories!