Proof of concept prototypes live up to their name - they serve the primary purpose of proving that a concept is valid. Such prototypes can be used to evaluate simple concepts such as handle shapes, part size, and feature placement. More sophisticated proof of concept prototypes can be used to evaluate the viability of much more complex mechanical or electrical systems.
Proof of concept prototypes generally do not look like a finished product. In most cases they are constructed with low cost, readily available materials. Despite their relatively low cost, POCs are a very valuable part of many product development processes.