Over-the-door-shoe holders are a way to compensate for the lack of storage space a "Standard" (Cat 11 & 12) cabin has (check out this thread I started on
Standard v. Deluxe).
The primary thing is that standard cabins have almost zero shelf space. While there is SOME space, there just isn't enough room for all the toiletries you're likely to need each day (tooth brush, toothpaste, mouth wash, hair brush, comb, razor, shaving cream, shampoo, conditioner, contact case & solution, dental floss, soap, Kleenex, hygiene products, sun screen, aloe for sun burn, hand sanitizer, etc).
The OTDSH can also be used to store small clothing items (socks, underwear, tank-tops, etc) to allow more room in the provided storage space for larger pieces of clothing.
If you have a Deluxe cabin, there is enough additional shelves in the bathroom and storage space for cloths the the OTDSH isn't REQUIRED, but is still a neat way to organize all the small stuff you've got to keep up with (camera, wallet or where ever you store your KTTW cards, batteries, film, keys, reading books, i-pod, hand-held video games, two-way radios, 1st aid kit, etc).