When I ask company leaders why they want to be registered to one of the ISO Management System Standards (MSS), I usually hear that the organization wants to demonstrate some level of consistency around the topic of the standard. Most often, one or more customers demand registration to a particular standard.
Back when ISO 9001:2015 was first coming out in the Final Draft level about eight years ago, I asked the owner of a medium sized company why he wanted his company to be registered to the Quality Management System (QMS). At that time, they had a number of customers but no one had the requirement for any form of ISO registration. Two larger companies did say the ISO 9001 was a preferred status for their supply base.