SSD is still expensive compared to HDD when talking about storage. HDD offers very cheap storage. Having a SSD allows people to have the benefits of the SSD: snappy OS, shorter boot times etc... But space can fill up easily for some people like games, music.
Getting a HDD allows you to out your media, documents there without filling up your 256GB SSD space which is very small compared to a 1TB HD.
Arguably if you have a NAS, you can simply only use one SSD.