When you create a tile dynamically, it is common to want to save it as a Png file so that it uses transparency (so that it respects the theme accent color).
However, there is no built in method to save a WriteableBitmap as a png.
There is however a nice extension to save it to jpg, but that doesn't help if you want to use transparency to leverage the theme accent color in your tile.

I think many apps that need to generate a tile dynamically will encounter this issue.