Unless you have a reason to keep people off your lists, there is no need to approve subscription requests. Of course, you may want to approve subscription requests for a variety of reasons. If your list discusses topics that are private in nature, whether in a business or personal arena, or that should not be seen by subscribers you do not know about, or could by used maliciously by an unscrupulous subscriber, requiring approval is an easy way to make sure unauthorized users do not end up on your list.
Listbox © 1996—2017. The Listbox Name and Logo are registered trademarks of FastMail Pty Ltd