I love doing ref sheets. I love visualising characters from just a brief description, or photos of animals, or... whatever. Getting to use my own creativity to mix characteristics from two species so that both are recognisable, while still keeping it true to the customer's wishes. It makes me really happy!
Also, They're nice and quick - instead of taking me painstaking hours to paint in lighting and detail (which I do enjoy, but in a more focused, technical way) they're flat colour, and I can usually finish them in a day if communication's good.

This is why I've kept their price at a flat $35 - I probably won't raise it for a long time unless my drawing skill improves DRAMATICALLY (unlikely... for a couple years probably!) so, yeah. If you want one done quickly, just ask :)