There were basically two hidden interests of KSA due to which she invested colossal resources in Afghanistan non state actors though this place was not in proximity to her
1 : KSA wanted to promote the philosophy of Wahabism in the entire region in general and CARs countries in specific and afhganistan could serve the best to achieve that because its a gateway to central asian countries
2 : Definitely Iran which has not good diplomatic relation with KSA and both are in race of countering each other agendas just like PAK vs india