An audience is a group of users or customers who meet given criteria. These criteria can include both innate properties (e.g., using an iOS device) and triggered activities (e.g., visiting a specific webpage). Audiences are useful for both measurement and ad targeting.
Building audiences for app install ad campaigns relies on exporting IDFAs and GAIDs. Without IDFAs, it’s not possible to generate a list of device IDs that can be sent to a third-party ad network (and even if you could, the network wouldn’t be able to do anything with it). This means iOS 14 and later eliminates the utility on iOS for most customers.
Until the GAID goes the same way as the IDFA, Android functionality in this regard is unaffected.