At first Sifo-Dyas was just a assumed name that Darth Sidious used to order the clone army. But George Lucas changed his mind about it, so the character was used in a story about the fall of Dooku to the Dark side.
Excerpt from Sifo-Dyas (Legends) article of the Star Wars wiki:
Biography
[...] Sifo-Dyas was once a good friend of fellow Jedi Master Dooku, and had the gift of precognition. Sometime before 32 BBY, he foresaw that the Galactic Republic would face a devastating war. To defend the Republic, he secretly commissioned a clone army from the cloners of the planet Kamino.
[...]
As a final test of his allegiance to the dark side of the Force, Dooku murdered Sifo-Dyas and took control of the project for himself. It was Sidious' true intention to use Sifo-Dyas only as a dead-end cover for Palpatine's plot to use the clone army.
Behind the scenes :
[...] "Sido-Dyas" was originally only a false identity for Sidious—Obi-Wan claimed never to have heard of him, and Mace Windu confirmed that there was no Jedi by that name.
[...]
His backstory was promised by George Lucas to be revealed in Revenge of the Sith, but instead is now explained in the lead-in novel Labyrinth of Evil.