swappo does a very specific task: it swaps one source object for one target object, with the new object taking on all of the target's animation properties. The target can be a single object, or a group.
One use of swappo is to create "meta particles" from the particle systems created with the excellent Primal Particles, by Primitive Itch. Simply create your particles with Primal Particles, using a simple object, such as a cube. Make sure you group the finished object. Now, select a metaprimitive (open the metaball tool, and choose a metaprimitive. Keep it selected) as the swappo source, and the particle group as the swappo target. Hit go and watch your metaparticles appear!
Another common use would be when working with coolpowers2 tools such as multiplex and particulator - i.e. tools that create multiple copies of an object. You could use low-oly objects originally, then swappo them out with higher-res meshes when it's time to render.