The primary candidate should be shown at the position of the cursor as you are typing. Some other Android keyboards do this, as does the iPhone.

You can keep your eyes looking at what you have written rather than keep looking down at the keyboard and the suggestions.

It makes for a much easier and more natural typing experience. I imagine this could be technically quite tricky to achieve but think there are clear benefits to speed of typing - SwiftKey's raison d'etre.