Same reason why there are bumps on the F and J keys...or raised dots on volume/channel buttons on tv remotes...
I don't think so. Because that's for blind people. And them being right and left, UNDERNEATH the big circle at the top makes that relatively easy, right? I think it maybe that, but definitely something else.
So the user forms an association between the shape of the button and its function, that way they are easier to differentiate in the dark.
Edit: I totally beat the stackexchange answer by 4 minutes :)
Is that really why? That's awesome
Well, I don't work for Apple and I'm not a professional UI designer. So I can't possibly be certain that I know the true motivation for that design decision. But I think it is a reasonable guess.