Historian: The early Egyptian pharaohs spent as much wealth on largely ceremonial and hugely impressive architecture as they did on roads and irrigation systems.
Conclusion
This was not mere frivolousness.
rephrase: It wasn't frivolous for early Egyptian pharaohs to spend as much on ceremonial/impressive architecture as they did on roads and irrigation.
Evidence (for)
If people could be made to realize the pharaoh's mastery of the physical world, their loyalty could be maintained without military coercion.
Evaluate
Whenever we do Role or Main Conclusion, we remind ourselves that 90% of the time the conclusion is the First Sentence or it's some Rebutal/Pivot claim.
This one is the latter, signified by however, and then confirmed by the fact that the following claim is prefaced by the Support Indicator ''for'. (F.A.B.S. = for, after all, because, since).
Goal
We're being asked about the role of the 2nd claim, which is our main conclusion. So let's go looking for Main Conclusion.