On the other hand, there is the I-It. The I-It relationship, is just the opposite nature. When we are not interacting in the I-Thou nature, then we are interacting with the world in the I-It nature. The I-It nature, even though has a value, it does not connect with us wholly. People do not recognize objects as they do others. For one, they cannot fully communicate with objects. The fact that we cannot is enough to understand that the interaction is not absolute. The I-It relationship allows the the subject to only understand the object for what it is. For the most part, it is the facticity that we recognize; we recognize the use of the object, and that it can be reproduced (for the most part).
Even though we can treat subjects as objects and vice versa, it ceases to be the I-Thou relationship. Because at the I-Thou relationship, there are no mediated forces that act between the subject-to-subject. It means that there are no influences and or other social ideas that divide the relationship between the subject-to-subject.
So how does this relate to God?
Well God is the ultimate. He is the absolute absolute relationship that we recognize. In this relationship, there are not any mediating forces between the I-Thou. Also, in the I-Thou with God, God is revealed to the individual directly. There is no need to find God. He is directly with the individual.
If you believe in a God: I believe that since God is the foundation for all other interactions, God is important in existentialism. God is necessary to define our human realities and to examine our world that we live in.
If you do not believe in a God: It is still necessary to understand how the others interact with the world; thus giving credit to some of the formations of our ideas and responses to the others' world (Even though I think this will be argued).
This is what I get out of it.
If I am interpreting Buber wrong, let me know. Also, please add on to it if you interpret it differently.