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.
The same options as feed-specific promotions apply. Here's a look at how it works when only one Instagram account is connected.
When you have multiple Instagram accounts connected in Spotlight, you can set up global promotions for each one by selecting the account from up top, as seen in the screenshot below.
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, as seen below.