In order for your engine to run properly, not only does it need a constant supply of air and fuel to burn, it also needs to be able to get the combustion exhaust gasses out of the cylinder. In order to do this while still allowing the combustion chamber to be sealed during combustion, valves are used.
Exhaust valves are opened and closed from the mechanical action of the camshaft and associated valvetrain components. Even though valves will last quite a long time, they can still fail, and when they do they will cause rough running conditions as well as poor emissions.
Some of the factors that can influence valve life are proper maintenance, especially oil changes as well as using the proper grade of fuel. Some of the things that can affect valve life that you cannot control are timing issues. If your timing chain breaks, or your timing chain tensioner fails, the valve can come into contact with the pistons which will bend them.
No matter the reason, if your valve have been damaged, they must be replaced in order to keep your engine running properly.