No, it's not a fashion accessory, although it is really cute :) It is a doggie backpack and there are several reasons for your dog to wear one on their walks. I first heard of doggie backpacks from my friend Jen. Her vet recommend it for the dog she adopted. I did some research and this is what I found....
Carrying a dog backpack gives your dog a job to do.
Most dogs need a purpose. Carrying their own backpack will give your dog a job. Not only will it help eliminate extra physical energy including pent up energy, but it also provides a mental challenge and gives a dog character. Pent up energy is the main cause of dog behavior problems such as chewing on things inside the home.
The dog will not be as focused on pulling.
If your dog is focused on carrying his own backpack, he will be less focused on pulling, being first and getting over excited about other dogs and people. It keeps the dog calm and focused on their job.
We put water bottles inside the bag and it added a little bit of weight :) Make sure to make it even though, the first walk with Sadie and her new bag I only put one water bottle on one side and she walked a little sideways, I think it threw her off. I read online to put no more than 10% of your dogs body weight.
There are other benefits, like you can carry water or gear on long hikes and they add visibility.
We purchased ours from Pet Smart and for the medium size it cost about $34.00. I saw them for less online, but I wanted to pick it out in person for quality and size to insure fit.
The brand I purchased is Outward Hound, cute name too :)