He needs to know. Abraham expresses his doubts, and he asks for clarification. He needed to know with certainty what God meant by His promise. Clarification is for the mind because our understanding is weak. Language can be ambiguous.
Words can have different meanings.
There can be equivocation. But I am childless. And according to custom, Eliezer of Damascus, my servant born in my house, is my heir. God answers with clarification. He responds in plain spoken words where there can be no ambiguity, no equivocation.