Talbot
"Who benefits and from what customs?"
Just off the top of my head:
The wives have their babies and are brought up in a family environment........that is a benefit that the Saudis seem to promote.
Their wives do not have jobs and adapt their lives to the environment which they are brought up to consider .."normal."
A hang up from the days that they lived as desert nomads and stayed at home ( tent) to look after the children and prepare meals for their returning husbands.
The wives are protected, as they see it, from the negatives of the outside world:
https://www.google.com/search?q=drunk+girls+saturday+nights&rlz=1C1CHBF_en-GBGB823GB823&tbm=isch&source=iu&ictx=1&fir=PoB4Sy2uHyF1RM%253A%252CG3TudffYg-GAOM%252C_&usg=AI4_-kTBteDFRSWFhaQp57xsv0RNNQZMIw&sa=X&ved=2ahUKEwjEsvjxmdnfAhWuRxUIHRC7AjQQ9QEwBXoECAIQCA#imgrc=d9fi-vpgvLrS9M:
Depending upon the status of the husband, they (the women) are endowed with worldly goods.....which may be important to some.
Quite a different culture from the West.