Google Sync offers some advantages over using IMAP on your device. Most importantly, it allows for content to be pushed to your phone within seconds after changes occur on the server. On the other hand, using Gmail Drafts from your device is not possible via Google Sync.
On iPhone, iPad, and iPod Touch devices, the Gmail web application offers the most comprehensive features and the best experience for most users. But for users who prefer using the native Mail application Google Sync offers an alternative way to access their Gmail account.