Theres loads more but it would be like trying to write the whole manual to do it justice. I think PSP is a capable paint program. Its on par with Gimp and PS for basic mapping stuff though there's bound to be additional features which it lacks. If I had to start from nothing I would go with Gimp but as it is I know this app well and I don't see any need to change. If you have it then its not a big problem to stay with it.

Its not good for everything but then neither is Gimp and PS. As Johnn said its not easy to scale and rotate tokens on a map as easy as other apps like mine, MapTool, or DungeonForge for example so if you have a LOT of that to do like we do in mapping then its best to take the images into that and run from that instead. You wont need to have zillions of layers to ensure that you can keep modifying the image. For strong vector work I would go with inkscape too but my style is not very vector based.