Fuzz testing is especially useful against large C or C++ applications, where any bug affecting memory safety is likely to be a severe vulnerability. It is these security concerns that motivate the development of most fuzzers.
Since fuzzing often generates invalid input, it is especially good at testing error-handling routines, which are important for software that does not control its input. As such, simple fuzzing can be thought of as a way to automate negative testing. More sophisticated fuzzing tests more "main-line" code, along with error paths deep within it.
Reproduction and isolation: As a practical matter, developers need to reproduce errors in order to fix them. For this reason, almost all fuzz testing makes a record of the data it manufactures, usually before applying it to the software, so that if the computer fails dramatically, the test data is preserved. If the fuzz stream is pseudo-random number generated it may be easier to store the seed value to reproduce the fuzz attempt.
Once a bug found through fuzzing is reproduced, it is often desirable to produce a simple test case to make the issue easier to understand and debug. A simple testcase may also be faster and therefore more suitable for inclusion in a test suite that is run frequently