In OpenMP any code inside a #pragma omp master directive is executed by a single thread (the master), without an implied barrier at end of the region. (See section on MASTER directive in the LLNL OpenMP tutorial).
This seems equivalent to #pragma omp single nowait (with the exception that rather than the 'master', any thread may execute the single region).
Under what circumstances, if any, is it beneficial to use #pragma omp master?
Though a single nowait construct is most of the time equivalent to a master construct:
The master construct can be used inside a work-sharing construct, should any need arise. This is not the case for a single nowait construct, as two work-sharing constructs can't be nested within the same parallel region
Some libraries want the main thread to perform certain operations. For instance the MPI library, when initialized with a level of thread support equal to MPI_THREAD_FUNNELED, allows only the main threads to make MPI calls