Manufacturers market socks for people that have foot sensitivities. They make them seamless, providing less irritation to the foot/toes and also I have seen an advertisement for a non-binding sock which would not restrict blood flow to the foot. I believe that they are trying to capture the market of diabetic people who are experiencing foot/toe problems. If your granddaughter's feet are not bothering her, it seems that this product would not benefit her in any way. Uncontrolled/High blood glucose levels contribute to diabetic people developing what is called neuorpathy or loss of sensation in their extremities so it is very important that your granddaughter and her family keep her blood glucose levels within a good range. Another good preventative measure is to have your granddaughter check or have her family check her feet daily to assure that she does not have any blisters or sores on them that may require attention. Your granddaughter is very lucky to have you for a grandmother.
Expanding a bit on EM's comment, children's socks used to be all cotton most of the time (but that 40+ years ago when I was a kid!), but now there're lots "fancy" socks that often have nylon or spandex. Some socks also have seams that can become friction points and cause blisters. The key is encourage her to wear socks that allow her feet to breathe & to wear properly-fitted shoes, sneakers and sandals.
It's a good question for her parents to ask her endocrinologist, tho' ... whether s/he recommends diabetic socks exclusively. They are often noticeably more expensive than regular socks, and for folks who need them, they are a blessing!
The benefit of all cotton sox without seams is that they are most friendly to our feet. They are most often recommended for folks who have a complications of long-term diabetes: lack of feeling in one's feet and poor circulation. At her young age, I hope she is still protected from such complications.
I have had diabetes for 7 years and recently worked with the diabetes educator at one of my local hospitals as a high school intern. We did a lot of teaching about diabetic socks. Diabetic socks are not a scam. People with diabetes are supposed to wear cotton socks so that the socks breathe. Also diabetic socks are supposed to help with keeping good circulation to the feet. Since people with diabetes need to take extra care of their feet. This would probably be a great gift for your granddaughter that she will thank you for when she gets a little and understands what they are.
Pumpgirl03