Re: Poor Uncle Owen and Aunt Beru... Oh Well!
Owen and Beru have to die so that Luke is an orphan Hero. Also, because it's why Luke is instantly a die hard rebel with a cause. Also, because someone who matters has to die horribly so that we understand the Empire is evil.
Or to put it in modern action movie vernacular, "Shit just got real
Baby, you and me were never meant to be, just maybe think of me once in a while...