Drives can be added or subtracted at any time so the system grows as your data grows and unlike traditional RAID you can mix and match drives of different types and sizes. Because it is not a real RAID implementation you can pull out any data drive in your array and read the files residing on that drive in another system. Data is protected against drive failure by single or dual parity drives and using SSD's for caching is even supported.
For anyone that is mildly technically inclined and enjoys DIY solutions, I would recommend at least checking out UNRAID before purchasing a Synology NAS (or similar).