Honestly? They don't matter as long as we can stick around long enough for the Emperor to nail Sariel to the wall.
To preemptively answer the invariable follow-up question to this, no, I don't know why Sariel lying or not makes any sort of difference in the grand scheme of things, particularly why Sariel lying makes conquering Gensokyo any less of an accomplishment. But given how he reacted, it must mean something to him for some reason, so I'm going for it.