1) They can make your payments as low as $0.00
Before you jump for a forbearance or deferment, make sure and see if you qualify for an Income Driven Plan. These plans can sometimes give you payments as low as $0.00.
2) They can bring your delinquent account current
If your account is delinquent and you are switching to a new payment plan, a Delinquency Forbearance is placed on the account. This will bring your account current until your new payment plan begins.
3) They may cost you money in the long term as well
For all the good they do, watch out for the extra interest. Paying over a longer time period or paying less towards your principal will cause you to pay more in interest and as a result more overall.
4) They are a better solution than deferment/forbearance
No matter what being in repayment is better than not. Deferment and forbearance time is limited and any interest that accrues will capitalize. So whenever possible, look to payment plans first!