Some exceptions make any sense only in the context in which they were thrown. This is particularly the case with, for example, the NullReferenceException, which may result from bad user input. In cases where you can't anticipate this by checking input in advance, you should catch the exception, and rethrow an ArgumentException to provide the caller with a better indication of the cause of the problem.

It can sometimes be a good idea to put a catch handler at the boundary of a component or design layer, to trap unexpected exceptions. In this case you might throw a custom InternalErrorException signaling the client code that something went wrong within your component.