Originally posted by BlakemoreYou ask weird questions that can't really be answered that simply, but I'll try.
Why write different versions of the same story?
Myths pass between cultures, which results in variations of different tales. Often historical events are slowly transformed into legend, which then alter as they are passed between cultures and generations.
A lot of modern stories are still the same as old ones, but they've changed context so much that they're indistinguishable. Storytelling isn't that complex unless it enters into experimental territory. This results in recurring 'plots' (sequences of events) and archetypes.
The King Arthur legend is not one story being told over and over, but is a collective sequence of retellings that change elements and add to the larger mythos. For example, the Grail was not always linked to Christianity — that was a detail added later, once Christendom had taken over Europe. The Grail became the cup that Jesus used in the Last Supper, and the lance (another important object in the Grail Myth) became the spear that struck Jesus' side when he was on the cross.
TL;DR: Myths and legends evolve over time as they are passed between cultures and generations.