When everybody's fully employed, breaking windows to "create work" simply diverts income from one place to another at the current moment - people spend to replace windows that they would have productively used elsewhere.

But if the government spends money hiring unemployed and underemployed people to do jobs, even so crude a job as dig holes and refill them (though of course that's not particularly efficient, as it doesn't build capital), it can either take that money out of the system at a later date (i.e. deficit spending) or inflate it away via seigniorage. The point being that the shift in spending is temporal. And that's precisely what you want in a recession.