Airgroup allows you to enable certain zeroconf device sharing features that are otherwise blocked by default on the EOU network.
On a small home network, there’s usually no need to block these services. However, at a larger scale, it becomes necessary for a number of reasons. The biggest reason, for most people is that some of these services become less usable if there are too many identical devics available. Imagine you want to Airplay to your Apple TV. You click the Airplay icon, and see 10+ different AppleTV entries in the list. Which one is the one you wanted? Who else can Airplay to your Apple TV?