Yes, you're right, head back is synonymous with go back, but is used more in colloquial speech or informal writing. Like 'C'mon guys, let's head back home' = let's go home Funnily enough though, it comes from a formal verb 'to head' which means 'to steer in a particular direction' which is why navigators of ships and boats talk about 'headings' when what they mean is 'directions'.