
There are any number of systems that have characters with multiple forms (White Wolf, D&D to name a few), etc. I normally keep multiple images on hand for my character (at home, adventuring, around town, fancy occasion; face-changing) and switching icons takes time. As a DM, it's also useful just as a marker for PC status to keep little "surprises" from popping up ("What do you mean you came to the royal ball with your demon armor and bleeding sword?").