Using orthographic projection means everything will be projected to a 2D plane and objects will not be affected by the distance and/or angle from which you see them. Look at the image and you'll understand:
The two objects are exactly the same, but they are projected differently.
Notice how distance affects what you see if you're using perspective projection. That gives the 3D touch to the engine. You don't need that in a 2D game.
If you don't want to include 3D in your engine you should be using an orthographic projection and an eye point (the point from which you are looking at the environment) above the ground.