Ahh. Good questions. Well Spiritualism is the ethos that is all about controling your population without aggressive methods like the purging and such that Collectivists like to employ. Spiritual pops tbemselves have ethics divergence reduction so once they are spiritualist they are hard to stray from the path. Converting other pops tends to be easy because of the other bonuses they can get. For example, the Divine Mandate spiritualist government type gives an empire wide ethics divergence reduction. This government type also gives a unique building that gives a 20% reduction on the planet it is built. Spiritualists also get a very good shot at getting the Symbol of Unity building which also reduces ethics divergence on a planet by 10%. If you upgrade all of this and make use of it, it's not difficult to get pops on a particularly troublesome claim to -60% divergence. At -60% divergence it really doesn't take long for pops to convert to your ethos, making them less likely to protest and usually making them more productive.

As for advantages over Materialsm, Spiritualists get the Symbol of Unity, the Spiritualist government types (in my opinion the spiritualist democracy and autocracy are some of the best governments in the game) plus the aforementioned easy pop control and they have a good bonus to unlocking the psionic technologies which is tremendously powerful. If you need a reliable method for keeping large, sprawling empires under control then Spiritualist is a good choice.