How does an investor earn money by buying bonds at a discount?

An investor earns money by buying a deep discount bond at a price below the redumption price (if it is an initial purchase) or market rate (if it is a market purchase) and holds the same till maturity or till a time where the discount gets reduced and getxs converted to profit. He may also wait till redumption where he gets the full face value of the instrument. He may some tiles get a premium or additional incentive like interest provided the initial offer document mentiones that and he is eligible for the same.