One of the problems I encounter occurs I'm not experienced enough to mix up just the amount of color I need, not even close, and watercolors are too expensive to wash away the left over paint.
So either I store away the palette I'm using, and just come back to it later or I find a way to use the paint. Since storing it means I need to keep buying new palettes, and half the time I can't remember what colors are on what palette it's better to use up the paint as soon as I can.
Sometimes, I don't like the mixes or just want to move on to different colors or there isn't a whole lot of paint. That's when I do something like this. If you don't mind a little smudging you can pretty up notes that you want to keep.