North Korea has prepared a ballistic missile for launch, so the U.S. is responding by placing the missile defense system on alert.
While the diplomatic dance to convince North Korea to back down whirls on, this shows that the US is regarding this as a serious threat without upping the ante to threatening offensive action. (A carrier battle group off the Korean coast would.)
It also shows the value of the missile defense system. This is the kind of threat the system was designed to defend against - a single rogue power led by a man who forgot to take his Thorazine this morning.
It gives us additional leverage - it demonstrates an unwillingness to back any farther while avoiding escalation.