How is streams different from Mastodon?
Mastodon allows you to build walls between server instances and lets total strangers intrude into your conversations - by default.
Streams allows you to combine server instances into a cohesive entity that is much larger than any individual instance through network-wide single sign-on, and protects against total strangers intruding in your conversation - by default.
With streams, conversations are the primary unit of communication and you control the audience and interactions of your conversations. Mastodon throws individual posts into space and the pieces need to be assembled from all over the planet if you want to follow the actual conversation. This may include total strangers and toxicity, spam, and abusive behaviour.
We're not saying one is bad and the other is good. Each has advantages and disadvantages. We just chose the default behaviour to be the one we find offers the most rewarding and less toxic online experience.
But the really big difference is that streams supports both ways of doing things.
Because we aren't in control. You are.