Connecting your public Facebook page to your Instagram account allows the brands you're partnering with on an Anchor storefront program to access audience insights and analytics from your organic and sponsored promotional posts. This enables them to see how your content is performing and how it relates to your storefront views, clicks, sales, and more.
A Facebook profile is not the same as a Facebook Page. Your Instagram account must be set up as a Creator or business account and connected to a public Facebook Page. How Do I Create a Facebook Page?