The importance of the 54th Massachusetts really is connected only to the race of its men. It is not important only for being one of the first black units organized, but all of its importance is connected to race.
The 54th Massachusetts did a number of things that helped to promote the idea of racial equality and to promote African American involvement in the Civil War. For example, their bravery in fighting at places like Fort Wagner inspired many more African Americans to sign up to fight. As an example of the unit helping to push for black equality, they all refused their pay for most of the war because the Army paid them less than it paid white troops. The 54th was also the unit of Sgt. William Harvey Carney, whose actions at Ft. Wagner were the earliest actions for which any black soldier won the Medal of Honor (though he was not awarded it until 1900).
So, the 54th is really important because of race. It fought bravely, but so did many white units. It is important because it helped to push for the idea of racial equality and it helped to show that African Americans were willing to fight and die for the cause of ending slavery.