Those who want an insight into traditional folk Islam in Indonesia and the impact of the radical version, will find this book very interesting.
Those wanting an understanding of traditional church life, might also find this book interesting. Most churches and many believers are syncretistic to some extent, and believe and practise things which are in some cases mistaken and in other cases just extra additions to the faith. Some of these don't do much harm. Others can be quite misleading. All can act as a veneer which hides what could be a different practice.
Does it matter? Yes. Can it be changed? With difficulty. Those who can distinguish the authentic from the additions don't always want to do anything about it. Why would one want to change something that has become comfortable and settled?
Some may want to change because the different practice might have power and benefit that is stifled by the comfortable and settled. Or because it is closer to what they think the Master called them to.
Dale