Well, first of all, because we all need ninjas. But Jonah Goldberg explains more specifically:
…it’s worth keeping in mind that we are already in an era of armed transnational religious movements. The problem is that they are evil. What would be so terrible about leading by example? Papal peacekeepers could do incredibly valuable work in parts of Africa or South America, protecting persecuted populations, delivering aid etc. The teaching effect could be profound (and perhaps very good at getting people back in the pews). Democracies are often slow and reluctant to do such work. And, if present trends continue America is going to do less and less of such things. Papal armies could do the jobs Americans won’t do.