Alright can someone explain the importance of the stainless steel stone?
It has many tiny holes in it that create many tiny bubbles that get absorbed by the wort, thus oxygenating it.
Without tiny bubbles, you're just passing air through the wort with minimal pick up.
Also, I almost always have starters for pitching. I use a magnetic stir plate. Does that mean aeration isn't as important?
Aeration is still important. There's a lot of discussion on other threads about stir plates and the degree to which they do or do not provide oxygen to the yeast. Regardless, they are not providing oxygen to the wort, only to the starter (if at all).
If you read the voluminous threads on the "Shaken Not Stirred" method, it's recommended there that you pitch your starter at high kraesun (I can never spell that right). I think this minimizes the need to oxygenate your wort, but does not eliminate it. Anyway, I haven't changed my starter practice so I don't know the details of SNS.