I believe it's merely for the ergonomic benefit. Especially when touch typing.
Imagine if you want to type "A" and you would have to use the right shift key. Try it for yourself. I would need my both hands, because I can't reach it.
You can find more information on wiki: http://en.wikipedia.org/wiki/Touch_typing
And also in a related post over at SuperUser: http://superuser.com/questions/304295/why-are-there-two-sets-of-the-ctrl-alt-and-shift-modifier-keys-on-the-keyboardhttps://superuser.com/questions/304295/why-are-there-two-sets-of-the-ctrl-alt-and-shift-modifier-keys-on-the-keyboard
Keep in mind that some of the "duplicate" keys (may) have other functions.