Unless you're under 5 foot or using a track saddle, you should still be able to mount from the ground - but you can use a sturdy little stool if you must. I would do this on good soft ground, like in a sand arena - it will make you feel less nervous about any accidents.
Or you are older
Or you have a round horse
Or you own a HUGE horse
Or maybe you just have more respect for your horses back and your saddle than others.
Sorry rant over, you don't HAVE to mount from the ground, I personally don't believe you should routinely mount from the ground.
Going to cool down now