These first started appearing on Commodore 64 and Amiga games that were distributed around the world. Initially they were simple messages, but grew more and more complex as the programmers from each cracking crew attempted to show how much more talented they were to other programmers in other groups.
Crack intros (or "Cracktros") started to feature big colourful effects, music, and scrollers. Cracking groups would use the intros not just to gain credit for cracking, but to advertise their bulletin boards, greet friends, and tell the world how superior they were. Messages frequently were of a vulgar nature, and on some occasions made threats of violence to software companies. Cracking crews often asked of the user to buy the game if they liked it though.
Programming crack intros became an art form in its own right, and people started coding intros without attaching them to a crack, just to show how well they could program. This evolved into the demo scene.
wikipedia.org dumped 2003-03-17 with terodump