I don't exactly why, but sometimes the Primary Sources tool give me the reference with the URL where is supposedly the statement confirmed and with the retrieved date, but most of the times it doesn't add a retrieved, so I have to add it manually. I think if Primary Sources tool could add always the retrieved property with the current date it could save time to all the users.

About if retrieved is or not necessary, I think yes, it is necessary, because it is very important to know when the user who add the reference access to the website or the identifier to confirm the statement.