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:
[...] 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.