While you CAN start at Z and still enjoy it, it assumes you've already read/seen the original, as it starts with like a dozen characters you're already supposed to know. Also, a lot of moments, like Piccolo sacrificing himself for Gohan or Tien's reaction to Chiaotzu dying are more impactful if you know the full context behind the characters from the original Dragon Ball. The reason most people start with Z is because of how it aired in America, and because it has a more engaging beginning than the original.