The primal essences of virgin witches could be said to be sources of incredibly concentrated energy in their own right. Furthermore, they would only accumulate with time until they were reaped or lost.
As such, Vaan believed there had to be more potent uses for the primal essences than to develop Specialized Magic and Divine Power.
After all, compared to Fourth Dream's primal essence, which had accumulated energy for thousands of years, the dream-attribute divine mana he collected from their dual cultivation would only be considered chump change.
More importantly, Specialized Magic and Divine Power weren't useful to him at present. As he reached higher levels, they would become even less useful.
Furthermore, they were also tedious to acquire.
He could just comprehend the relevant laws to develop the desired abilities and divine power.