The phenolic spacers keep the aluminum from making direct contact with the hotter cylinder heads, thus keeping them cooler for a cooler air charge. Just how much cooler?
The intake sits above the hot engine, so it will eventually get hot anyway, though probably not quite as hot with the spacers. I'm not convinced of the spacers total effectiveness. The bolts will still conduct a fair amount of heat.
As I finished up the job, I was concerned about getting everything lined up during reinstall. The spacers would have complicated that, especially at my relative lack of skill level.