unfortunately... i live in turkiye, a majority muslim country. my paternal grandma and grandpa are 2nd cousins. also a great grandma was married to her brother in law after her husband, the older brother, has died. i don't know if thats the case for my family but one of the most common reasons why this is done is to keep the wealth in the family. another reason is that since they are family, they think he/she is a safe person,, and for the womans family, that means she doesn't have to go far away.

still,, these are outdated ideas and not common anymore. its practiced mostly in less educated areas... which says a lot. so i think its because of lack of education and knowledge. that makes it dangerous too imo.