Your name and nickname; and the names, nicknames, and relationship with you (for example, “my dad”) of your address book contacts (collectively, your “User Data”).
To be fair it is used to help the dictation process.
All of this data is used to help the dictation feature understand you better and recognize what you say. It is not linked to other data that Apple may have from your use of other Apple services.
If you turn off Dictation, or switch from using server-based Dictation to Enhanced Dictation, Apple will delete your User Data, as well as your recent voice input data, from Apple’s servers.
So if you turn it off, it deletes that data.
Older voice input data that has been disassociated from you may be retained for a period of time to generally improve Dictation, Siri, and dictation functionality in Apple products and services.
This voice-input data may include audio files and transcripts of what you said and related diagnostic data, such as hardware and operating system specifications and performance statistics.
And it can be used for improving their dictation service.