Why do we have node facets?

A brief detour to talk about motivation. This example is all very well, but why, in general, do we have the concet of node facets?

The answer is that it makes it very easy to extend the node's functionality. Rather than extending the node's own code it allows us to write a netlet to do this, acting as plugin. Other netlets can use this node facet. Further, the providing netlet can impose permission-based access to help control use.