Resonance was the greatest gift and hitch to Rowan's upgrading of his Angel's Ranks. Without Resonance he would only have access to Angels, powerful forces to be sure, but not enough to stand outside the universe, with Resonance he could increase the Ranks of his Angels.
The simplest method to understand this was that an Archangel required the merger of two Angels for it to be created, and to create a Sovereign it required a merger of seven Angels, but there was still another restriction to this process.
Rowan had hundreds of millions of Angels currently and if it was as simple as merging random Angels to get a higher-ranking one, he would already have Thrones and Dominion in his Great Hosts.
What was needed was Resonance, as every Angel was unique, and to merge any two angels, they would essentially have to be the same.