Division of hosting requirements, one Thingshare instance won't need to host every user or every uploaded file
Safe experimentation, one instance can try new things without significantly affecting other Thingshare instances, but still have access to the same content
Your instance your rules, if someone wants to make a 100% SFW Thingshare instance they can. You can choose to only federate with instances known to properly tag NSFW content and filter it out, etc.
Thingshare's federation builds on JSON-based RESTful RPCs, which are also used for local content where applicable, reducing code duplication and "dogfooding" one's RPCs.