I think there are some misconceptions here. ActivityPods does not build a new standard but actually implements existing ones instead (ActivityPub and Solid).

So if you are a dev, you can write your own app on top of ActivityPods and it gives you the ActivityPub support almost "for free". This gives you: (1) freedom of the users where they have their account when logging in to your app (like a sign in with google) but the user data is stored with the user's personal online datastore (POD) not with the app (2) the possibility to deploy multiple instances of your app which can be useful to build communities independent of the account provider. Also (3) as an app dev, you don't need to worry where to store users' data.

For an existing ActivityPub application (take mastodon for example, we have an alpha stage app called mastopod which is intended for microblogging), there is nothing to do actually. If someone builds an app that supports the same types, for example the Article or Event type of ActivityPub (/ActivityStreams), they can understand each others activities. You could for example imagine an app that functions like an aggregator for all types of Activities and all people you follow but you might want your blogging app to only show you Articles. And another app that is nice for organizing events.