Killing a Primordial was rather anticlimactic, but it was not as if Rowan was expecting much. After all, what he had just done could be seen as cutting off the hand of a massive creature with a thousand arms.
The blow had been painful, and may have lasting consequences to the Primordial, which may most likely end up being devastating in the future for that entity, but that was not the most significant usefulness from her death.
Every Primordial inside of Reality was intrinsically connected to Rowan, and it was by killing them that the connection they held with him would be revealed, and this was equally as important to him as any growth in power he would gain by battling and killing them.
So the Primordials for him were the keepers of his past, and for Rowan, the past was like a book; killing Primordial Soul was the first chapter to that book. From her, the connection would begin to unravel.
