It's actually far more simple.

Without the outbox model, you have to keep adding more and more relays that you read from so you will see the posts from all your followers, and more and more relays that you write to so that everyone who follows you will see your posts.

With outbox, you can have a handful of read relays, a handful of write relays, and other users' Nostr clients just read from whichever ones you wrote to and write to whichever ones you read from. You as the user don't have to do anything to make sure that you can see the posts those you follow are making. Your Nostr client just looks up what relays they are writing to and shows you their notes, even if you don't have them in your relay list.

It's delightfully simple when properly implemented.