The Prophet Muhammad (peace be upon him) had a beard and he ordered the Muslims of the time to grow one too. Back then it was important for Muslims to look different than the Pagans for their security and the beard served as an identification element of a Muslim.
Also, the beard served as a new tradition that helped the newly-converted Muslims give up on the Pagan traditions they grew up with.
Muslims of today grow a beard by choice because it was a Sunnah (tradition) of every one of Gods prophets and messengers that was sent. From Adam, Noah, Lot, Abraham, Islamil, Isreal, David, Moses, Jesus, Muahmmad etc (May Peace Be Upon Them All). In doing so they are emulating the best of mankind and those who were the most obedient to God all mighty.
Further more growing a beard earns Muslims a steady residual supply of good deeds, in particular when they are following the tradition of the final Prophet Muhammad (Peace Be Upon Him) as a sign of faithfulness.
However, it is not mandatory for Muslim men to grow a beard and failure to grow one is not a sin of sorts.