This question goes back to Thomas Aquinas’ Five Ways. One of those ways is that God is a necessary being. He is the only being that exists in Himself without anything else, a-sē, which is what aseity means. God exists in Himself as a necessary being, not a being that needs something else or what we would call a “contingent being.” You and I are contingent beings. We need prior being so we can come into being. God is outside of the chain of being. He is a necessary being.
The aseity of God was so fundamental for R.C. Sproul. I remember him getting excited, lifting himself out of the chair, and saying, “That’s the key, the necessity of God. That’s the key to the five ways.” Aseity is important because it really gets to the essence of God, which is what Dr. Sproul was trying to do in his book The Holiness of God. He used the word holiness because there are all kinds of drama attached to it. However, he was talking about more than an attribute. He was talking about the essence of God in His being.
Dr. Sproul wanted the church to understand Reformed classical theism fully, and I think that’s why he focused on the aseity of God. There are a couple of pages where he described aseity in The Holiness of God, but he would have liked to write a book about it.