When the hamstrings are tight, the arms are often not long enough to reach the feet. This is a problem in poses where you have to grab your toes, such as forward bends. Instead of bending the legs, you can loop the belt around the feet. Belts can also be used to bind the legs together in restorative poses such as Supta Baddha Konasana (Reclined Bound Angle Pose).
Instead of a yoga belt you can use an old neck-tie, a dress belt or even the belt from an old bathrobe.
My personal favorite in belts is the "Pune" belt, as used at RIMYI, the Iyengar Institite in India. These belts only tend to be available from Iyengar yoga specialist stores, such as Tools For Yoga or the Iyengar Institute in San Francisco. It is a thinner belt with a smaller buckle and is generally more versatile and less cumbersome that standard American belts: