Why the cover? To protect the innards.
Why a half-title? It's a hold-over from when there weren't covers.
Why cloth? Cloth is a damn fine material in which to wrap stuff you want to protect.
There is a tremendous opportunity for book designers and software engineers to figure out what our digital book procession should be. There's clearly something lost when you're thrown into a text without context — but how should that context be delivered? What 'function' should a cover serve in the digital book space? And even: What is the cover?