Currently all Authorization requests are directed requests. The Authorization server must ask the user who they are unless they have an existing session.
Sending the id_token in the request provides evidence to the IdP that the user has previously logged in. Allowing the IdP to prompt the user with the correct ID or directly return a response if the user has persistently authorized the scopes for the client.
Currently if the user is logged into site A with ID User_1 but now logs in to the IdP with a second account User_2 and the IdP performs a request to the authorization server with prompt=none the IdP might reply with the ID User_2 if it has been used there before and perhaps leaking information about the linkage of the accounts.
I am uncomfortable about sending prompt=none without a user_id. I think it has uses being optional on all requests, but required for prompt=none.
This is equivalent to openid 2.0 openid.mode=checkid_imediate, That required a claimed_id in the request.