Q: apple id and email address

You can't change it to not be an email address, you can only change it to be another email address - when you change the id an email is sent to the new email address so that you can verify that you have access to the account (and receipts for purchases will be sent to that email address)

For music you can create a new version of the tracks after downloading them, that will remove the id : iTunes: How to convert a song to a different file format

You can't do similar for other content

You don't have to keep them, but it's generally a good idea to keep a backup copy of all of your purchases (e.g. on an external drive) - there is no guarantee that you will be able to redownload any item from the store, the rights-holders could remove them at any time