[Oldbayrunner]

Because there are times when you RUN OUT of battery power but still need the camera. AAs allow you to pop into any convenience store on the planet (near enough) and be good to go for a little while longer. That can be long enough for your camera battery to gain sufficient charge.

As I stated previously, this is another reason to select grip first, if your grip runs out of battery power that gives ample time to A. either replace that battery or B. put another in the grip. Regardless when it switches to the camera battery one has at least another 900 or so shots left. By the time you spend enough double A's to equal an LI 90 you could have purchase another off brand or so to keep with you IMO.