I think it exists because it's a concise way to communicate "this is ending, and will end in a predetermined amount of time, but it has not yet ended" in the same manner that sunset indicates that a day is ending but has not ended yet.
I realize this usage is becoming an official usage, but the original meaning was something that would end by design and automatically unless it was renewed. Like a law had a sunset provision if it already had an ending named at the time it was created. So the ending is built into the original plan from the beginning, it's bound to happen unless something changes.
Like... a sunset.
It's not something passive that's happened at Google though. They're saying we are going to close a service that is open, and has been expected to stay open indefinitely. It's not a thing that is just kind of happening, it's a decision that was just made and is being newly announced.
And even knowing what it means, as I do now, I wouldn't say that sunsetting is any clearer in communicating that it's "going to" end than "shutting down" or "stopping" Google+. It's not as if the proposal said "ended" or "shut down" Google+, the "are" in something like "We're shutting down Google+" signals continuation to me.