Such as making sure our ports are safe from some freighter carrying a nuke. Sticking our noses into every meddlesome neighbors behind and choosing sides is making us the world's bully I love to hate. You international busybodies should look around you and solve some of the problems locally instead of searching the world for problems. There are plenty here.
As to why we need peacekeeping forces, it's easier, cheaper and more efficient than allowing things to get so out of hand they end up here. Or do you like having all the worlds refugees seeking asylum here? That's what happens when people can't live in their own countries, and we accept them. How about approaching problems in a more cost effective manner.