To take care of her own body. Sure, cleaning, cooking, playing, and all that is a part of the job, but your own health should be at or near the top of the list.
I don't buy ANYONE'S reasoning of "I don't have time to exercise or eat right" as an excuse, but especially SAHMs. You should be able to carve time to focus on your body throughout the day, or incorporate it into your lifestyle with ease.
*of course, barring TRUE medical conditions...many of which can be reversed or at least maintained through a raw food diet*
ETA - this obviously should apply to everyone, not just SAHMs, as the original title said... it's just that I see the "I don't have time" line most often from SAHMs. I promise if you have time to do everything else, you have time to treat and fuel your body correctly.