The "point" to a "sup" is one trying to say hello, sup was derived from wassup, which is another variation of What's up. What's up isnt an inquiry but an informal way of saying hello, although it can at times be answered with an actual response (Ex 1.)
Many do in fact actually respond to the question with an answer but quickly disregard it and fray away from it... another way of responding to it would be (Ex 2.)
Ex 1.) "Sup matt?", "Nothing much just watching gay porn on my computer"
Ex 2.) "Sup jew?", "Hey man, havent seen your ass in a long time... wanna hang out on Saturday?"