Kapitel 748: How? Why?

At this point, about the only thing Ryu's Realm Heart was good for was protecting his heart should it be stabbed through. After entering the Immortal Realms, there was simply little else that a Mortal Realm Realm Heart could help Ryu with, especially since he had already reclaimed the Phoenix Sky God's Spiritual Foundation so he didn't need to rely on it to cultivate without a dantian. 

The truth was that the Realm Heart had so many abilities that Ryu had just never used because he outgrew the Mortal Realms so quickly that they were hardly useful. 

For example, in terms of comprehension, a Realm Heart allowed for a grasp of knowledge even beyond Ryu's Heavenly Pupils. And, when the two were combined into one and used as one, the speed of comprehension could be considered to be obscene.