> Why do strings cache their UTF-8 encoding?

Mainly for compatibility with existing C API. Common way to parse function arguments in implemented in C function is to use special argument parsing API: PyArg_ParseTuple, PyArg_ParseTupleAndKeywords, or PyArg_Parse. Most format codes for Unicode strings returned a C pointer to char array. For that encoded Unicode strings should be kept somewhere at least for the time of executing C function. As well as PyArg_Parse* functions doesn't allow user to specify a storage for encoded string, it should be saved in Unicode object. That is not new to PEP 393 or Python 3, in Python 2 the Unicode objects also keep cached encoded version.