Sometimes, just to see how pretty my town looks in 3D.
I tried the new 3DS which my friend owns and now I kinda wish I could get one for myself, because the stabilization really helps a lot. With my old 3DS I have to make as little movement with my hands as possible to retain the proper 3D effect. It's pretty easy with calm and slow games like Animal Crossing I guess, but with action games it's almost impossible.