a) when a situation is so uncomfortable, you'd feel almost less awkward if someone was sucking your butt. Granted, the butt would be the least favorable of places on a human body, any body actually, to suckle.
b) to express extreme distaste or misfortune in a situation.
c) to get people's attention. (With such a profound remark, your hopes, desires, questions and concerns will all be met by this simple phrase.)
Sucks my butt! I couldn't find the light switch in your bathroom and I really had to take a dump...where's your broom?
Frank: SUCKS MY BUTT! I can't find the chips!
Hot Chick: BBQ?
Hot Chick 2: Cheddar?
Hot Chick 3: Salt & Vinegar?