"Ah. I see why this is a think."
Umm no you don't. If you have a relatively static requirement for storage (i.e. it's not growing by TB's every month), then a simple NAS or Fileserver is sufficient - These are intended for companies who hoard data (because it "might be useful one day").
The purpose of it is to provide fast local storage, but remove the size limitations and complex backup requirement - If you purchased a 10TB NAS, then exceed this, you either have to purchase a JBOD or a whole new NAS. With Nasuni style devices, only hot data is kept locally, in addition, you can forgo the requirement of back-up infrastructure (tape, dedupe etc) and make use of S3 and Glacier.
Another use case (at least for similar appliances) if you are a globally distributed company, you can have multiple appliances connected to the same S3 buckets - so they effectively provide a Global DFS and local cache.