Snape did this on Dumbledore's advice. By moving him early, but deliberately leaking misinformation to the Death Eaters, Snape was exerting a measure of control over the situation, thereby increasing the odds that Harry would be able to be moved safely, while preserving his cover as a Death Eater.
"You will have to give Voldemort the correct date of Harry's departure from his aunt and uncle's," said Dumbledore. "Not to do so will raise suspicion, when Voldemort believes you so well informed. However, you must plant the idea of decoys; that, I think, ought to ensure Harry's safety."
- Deathly Hallows, Chapter 33, "The Prince's Tale"
While the plan was risky, so were any alternatives. As was noted earlier, Voldemort had people watching the Dursley's. By leaking the information, Snape was controlling the Death Eaters' response as much as possible.