It’s more a UI/UX thing to ease configuration. In ID3 the POPM frame consists of play count, rating and “e-mail”. The original idea was that one could store multiple ratings for different users in the tags; hence the “e-mail” field to distingiush the users. It is called “e-mail”, but it really is just a string to distingiush between different ratings in a file.
This didn’t really get used, but players started to save their rating there with a player specific string. E.g. Windows uses “Windows Media Player 9 Series”, MusicBee uses “MusicBee”.
For Windows Media Player (WMP) you see it was originally done for version 9. But they realized they could not change this name in later versions without breaking compatibility, so it stuck. And because WMP used to be quite popular some other tools just use the hardcoded value “Windows Media Player 9 Series” as well. If you want Picard to write ratings that can be read by WMP or a compatible player you need to set “Windows Media Player 9 Series” for the e-mail field in the rating options.
Now my idea was that you could select between pre-defined settings, which are setup for specifc player compatibility. That way there could be a WMP preset, that automatically uses the proper value for “e-mail” and also uses a mapping that is WMP compatible. Likewise there would be a MusicBee preset etc. Even the current Picard behavior could be available as such a preset, and I’d like the ability for plugins to add additional presets.
You could even enable multiple profiles and store the rating in a way that is compatible with multiple players. I even think we could make use of storing both your personal rating on MusicBrainz and the overall rating in the tags, currently you have to choose. But why not store the community rating as “community@musicbrainz.org” and your personal one as e.g. ernstlx@musicbrainz.org.
The following thread also has some background on how Picard handles ratings and some thoughts of mine on the topic: