When defending the realm against raids, you'll earn Glory. However, you'll not be able to use that Glory until you cause Ragnarok.
Once you cause Ragnarok and gain Glory, you can use it to upgrade your relics in the Yggdrasil. For example, upgrading the Gjallarhorn will dramatically increase the amount of Iron you'll earn, allowing you to continue further into the game than you've been able to prior.
Glory is also required to upgrade Champions alongside their duplicates.