If you're a full time artist (and don't have a diligent assistant/spouse/significant other who keeps track of these things), chances are you can quickly lose track of matters like where your paintings are, how you titled them, what size they are, how you framed them and when you varnished them. You can also lose track of where photos of your artwork are stored (this is particularly applicable to those of us who started our careers in the pre-digital photo age).
There are many systems for cataloging your work, but the one that I'm using now and like very much is Artwork Archive. It's simple to use and has a couple of options in terms of fees and number of images you can post. I feel extremely organized having this cataloging system.