As well as for security, I believe it's to help with things like purchase history, recommendations etc. technically even though it's free, it's still a purchase

As well as for security, I believe it's to help with things like purchase history, recommendations etc. technically even though it's free, it's still a purchase. If you change your phone it's easy to back up or remember the apps you had installed by referencing purchase history and stuff. I'm not sure also but if something goes from being free to paid, you are covered to still get it free but that is purely speculative.