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 the same kind of access so it's not a bare test with nothing but test items
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. (however tag support is still lacking at time of writing)
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.