Mobile devices are slow when it comes to paint pixels on the screen. So the less pixels the application draws, the better. It becomes worse on tablets with giant resolutions.
In 2D, you are going to have a lot of images to draw. drawing a sprite actually means drawing a rectangle with a texture on it.
However, transparent textures include completely invisible pixels, but they are STILL pixels to draw. So the idea is to use a better geometry.
It's possible to outline blank areas manually, but it's a repetitive task, and I'm not even sure TileMap supports polygon nodes as tiles. And how would you animate that out of the box?
So if the engine can precalculate that outlining and do the work automatically, like Unity3D does, it would be both a performance gain and time saver.