Instant Audiences provides events for all identified traffic, which includes site traffic for emails already in your email database.
This enables the Audiences flows to re-engage dormant profiles that we can identify and track which Klaviyo can not. Audiences is able to provide events for profiles indefinitely as soon as we identify them, whilst Klaviyo only receives signals from your Shopify store if a user is logged in or has logged in within the last 7 days (for iOS). This means that Audiences is able to provide signals to Klaviyo for a larger number of customers - both existing and net new.
The Audiences flows will not send emails to customers that are already tracked by Klaviyo - flow filters will prevent this from happening.
This means that if a customer received an email from an Audiences flows, then Audiences was able to identify and track that customers website traffic, whilst Klaviyo could not.
This means the customer would not have been re-targeted if Audiences was not enabled.