Let me explain how it works when you self host like me:

• “All” starts out completely empty, there are no federated instances to find this way.
• You then have to browse communities on other instances and subscribe to them on your own instance. Only then will posts start showing up in “All”.
• Since there’s only 1 user, the list of communities in “All” is the exact same list of communities in “Subscribed”

For most people yes, you can just browse “All” unless you’re on a smaller instance, since someone on your Instance has probably already subscribed to the community you’re looking for.