The explanation is cute, but fairly non-obvious (from Evolution of a POE Server, on the POE site):

The POE based server we just completed is still a bunch of work. What's worse, most of the work never changes from one server to the next. Listening to a server socket and accepting connections from it is a well-established science. Likewise, performing buffered operations on non-blocking sockets is largely the same everywhere. Reinventing these wheels for every server gets old very fast.

We created a group of classes under the POE::Wheel namespace to encapsulate these standard algorithms. Each Wheel contains some initialization code to set up event generators, and each implements the handlers for those events.
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Mike