The two different encodings aren't just for config files; property lists are used throughout the OS for all sorts of things. They're a serialization of standard data structures such as arrays and dictionaries: same idea as JSON, decades before JSON. In retrospect, the XML-based text format is excessively verbose, but back then XML was all the hotness… As for the binary format, well, even given JSON's relative succinctness, there are many different formats that attempt to fill the role of 'binary JSON' (BSON, BJSON, JSON-B, MessagePack, CBOR, UBJSON, Fleece, PSON). It's just that none of them have become ubiquitous, probably because of some combination of fragmentation and the lack of built-in browser support.
As for the daemon, among other benefits, it allows preference changes to be immediately visible across all processes on the system, rather than requiring manual action to reload from disk. See:
