The fact that we have new information suggesting they are soon going to have intercontinental ballistic missiles, capable of hitting the United States. They don't have enough to destroy us, but definitely enough to kill millions. Now it's more important then before to get them to stop progressing in nuclear technology. Whether you agree or not, I don't like the idea of them being able to hit the United States.
He has attempted to speak, which they refuse. Negotiations have also been attempted, and Trump is definitely working with China to resolve this peacefully. Just because he's using a show of force doesn't mean it isn't with the intention of peace.
Actually, in the event of North Korea deciding to start blowing everyone up around them, it would be wise to have something that wouldn't take 10 hours to get there, even at 700 mph. But I don't think that's the idea.
In the event of a preemptive strike, there would be two options.
Destroy North Korea entirely, wipe it off the map.
Or, do surgical strikes and eliminate most of it's long range military power before they could cause much damage.
Obviously, the second is a much better strategical idea because the first could get us into a long term conflict in Asia.
Doing the surgical strike, first we'd have to position stealth planes, carriers, and navy ships near North Korea. This way, with in and hour or two, we could effectively destroy most of North Korea's nuclear weapons, artillery, and a good portion of their infrastructure.
What better way to sneak in ships then to pretend it was just a show of force, and when you did get too close, retreat back a little bit. Doing that, you would "admit" it was a bluff. But in actuality, you just managed to position a bit more firepower in the region.
All depends on what Trumps ultimate goal and strategy is.