Yoga belts are sometimes called yoga straps, and they can be a massive help if you're not quite able to achieve certain poses. If you can't reach your feet during a seated forward bend, a strap around the soles of your feet can give you the extra length you need to stretch out your hamstring muscles, getting you closer than you'd be able to without a prop.
Getting rid of some of the frustration if you're not quite able to perform an asana makes your practice more rewarding, and lets you benefit much sooner. Keep a belt in your mat bag and you'll be able to use it whenever you need a little extra assistance.
Yogi at any level from benefit from using a belt, getting deeper stretches by the help this versatile tool provides. The versatility of a yoga strap also means that it's suitable for use in pilates, as with equipment like a sand bag.