Why does Obama need the power to take over state National Guards whenever he wants?

Up until now a president could only takeover a state's national guard during a time of war, isn't it a big change to say that the President can now take over the national guards at any time? Why is that part of the health care bill?
The constitution limits funding the military to two years at a time. This legislation sets up a domestic military that can be funded at four years at a time.
Judge Napolitano asks: Why do doctors in this new division that is being set up need to have uniforms and guns?