This is the first piece of support equipment I purchased for my own practice. A strap helps me to be able to safely and more effectively stretch my hips and shoulders in some poses like Happy Baby and Seated Forward Fold.
I also use it to keep my mat securely rolled between sessions!
The length you want will depend on your height. I still have a beginner's 4 foot, but I wish I had a 6 or 8 foot one. If you're over 6 foot, definitely go for the 8 or 10 foot length.
I am working on becoming a Gaiam affiliate. Stay tuned!