Buying a full server is both expensive to buy and to maintain. As NAS devices became popular they give us the chance to have servers that are easy to acquire and cheap to maintain especially when it comes to power consumption.
It is also an overkill to get a full server for a personal or low-traffic website so if someone would like to host their own sites but not in a professional level or without a hosting plan (LAN only usage or home automation) then a NAS could be a good solution between a server rack in the closet and a service given by a hosting company, especially that a NAS is designed to be power efficient, easy to maintain, silent, to be run 24/7, not to mention the support for RAID.
What are some cheap NAS devices that would make a good webserver that can serve a maximum of a few hundred visitors a day and provide the freedom of a regular server but in a smaller scale. Something that let you install different technologies used in web developement (not just PHP and MySQL). Possibly give SSH access and has an operating system that is (in term of possibilities) the closest to a "normal" server OS like Debian or CentOS.
Low cost means it is cheaper than a desktop PC used by the average, everyday user in First World countries.