Why a hybrid MPI/OpenMP programming strategy?

This year, Moore’s Law turned 50. The law, formulated by Gordon E. Moore, co-founder of Intel, suggests that “The number of transistors incorporated in a chip will approximately double every 24 months”. Until now, the law has always been validated. But, since few years, depsite the fact that the law is still valid, the processors wall-clock frequency seems to stagnate, and the law reaching a limit, mainly due to prohibitive electrical consumption.