Aside from being able to set up promotions for individual Instagram feeds in Spotlight, Global promotions make it possible to apply promotions to each individual Instagram account's posts.
When you have multiple Instagram accounts connected in Spotlight, you are able to set up global promotions for each one individually by selecting the account from the sidebar. To apply a promotion to a post, simply click on it in the preview area, select the promotion type in the popup modal, and choose what to link that post to.
Promotions applied through the Global option, found in Instagram Feeds > Promotions > Global, will be applied to all individual feeds unless explicitly removed. When setting up or editing a feed, under Promote you will see the option to disable Global Promotions in the feed editor's sidebar.