Durable Streams are a lightweight network protocol on top of standard HTTP. When you are building a synchronisation layer for let's say a local-first app, you need to not only exchange data over some lower-level protocol (i.e. HTTP / SSE / WS), but you also have to define a higher-level protocol on how the client & server are going to communicate - i.e. how to resume data fetching once the client reconnects, based on the last data that the client received (~offset). Since the reconnect & offset should be automatically handled by the Durable Stream, you could just build your domain logic on top of it.

CRDTs are primarily meant to resolve data conflicts, usually client-side, based on a defined conflict resolution strategy (i.e. last-writer-wins). Some of the CRDT libraries, like automerge, loro or yjs, also implement a networking layer to exchange the data between nodes (could be even P2P), meaning they already have a built-in mechanism for reconnection and offset (~send me data since X). However, nobody forces you to use their networking layer, meaning that with Durable Streams, you would have a good starting point to build your own.