I don't believe that there is a "why" to use them, or a "why not." The DMG provides guidance on how to say change the religious system from polytheistic into say Monotheism. Changing it, though, isn't something that you either should or should not do. It's there to give you guidance on the process if that's what you as the DM want to do.
Presenting the guidance neutrally like that is appropriate, because it doesn't unduly influence DMs(new or old) to run the game in a particular manner.
There is also guidance for new DMs right at the beginning. It's on page 9 under Core Assumptions. It lets the new DMs know what the default dials for the setting are and explains what those dials mean in broad terms.