Explain Dursleys departing in Harry Potter the Deathly Hallows.?

The Dursleys leave for their own protection, because when Harry turns 17 the protective charm at their home will break, even if Harry is gone it is likely that the Death Eaters will capture and torture them for information on Harry, as they would neither be particularly aware of, or care about, Harry's lack of a communicative relationship with the Dursleys.