Reason #1: It's funny.
Reason #2: Strong Bad has an outrageous accent
Reason #3: It isn't every day you get to see gummy bears in milk.
Reason #4: Lady-ing
Reason #5: Trogdor! Trogdor!!
Reason #6: Cheerleader, So and So, What's her face... The Ugly One! Soooooooo goooooooooood...!!
Reason #7: "Virus = Very Yes"
Reason #8: "On a scale from 1 to Awesome, I am Super Great!"
Reason #9: "The critics agree! "A triumph!" "A cinematic breakfast snack of some kind!" "It {very slight pause} sure was a movie!" {quickly} Now playing in select cities. {rapidly, in the manner of a disclaimer} This Strong Bad Email has been rated P for The Poopsmith and a graphic scene of The Paper."
Reason #10: The Bellydance Superstars love the Web site!
List largely courtesy of Elena. Links added by me. For all things Homestar, visit the wiki. That's wiki, not wookie.