The site brings your Twitter and Jaiku accounts onto one screen and adds a posting interface that allows you to post just to one of the services, or to both. The benefit for many people that use both services religiously is obvious. Very simple and very useful for some people.
Both Twitter and Jaiku have APIs, making this possible (or at least manageable). Clones/similar products like Pownce and the new Yappd don’t have APIs. When and if those services release them, I’d expect TwitKu to quickly add those services as well. And that would save those of us who want to use all of the services but refuse to choose a lot of time.
And since Twitter and Jaiku are all about presence and status updates of friends, there’s no reason not to add Facebook status right away, too.