For the same reason you would use one in enterprise deployments: if setup properly, it's easier to scale. You don't need to invest in a huge storage server upfront, but could build it out as needed with cheap nodes. Assuming it works painlessly as a single node filesystem, of which I'm not yet convinced if the existing solutions do.
Not really. Most consumer motherboards have a limited number of SATA ports, and server hardware is more expensive, noisy and requires a lot of space. Consumers usually go with branded NAS appliances, which are also expensive and limited at scaling.
Setting up a cluster of small heterogeneous nodes is cheaper, more flexible, and can easily be scaled as needed, _assuming_ that the distributed storage software is easy to work with and trouble-free. This last part is what makes it difficult to setup and maintain, but if the software is stable, I would prefer this approach for home use.
