Why were the U. S. Colored Troops so vital to the Union?

Black troops made an important percentage of the total Union Army force, around 10% of the Northern armies, according to historians. Some 180,000-200,000 African American men - some former slaves, some free before the war - fought in the Union's ranks between 1863 and 1865. Only after the Emancipation Proclamation of 1863, military service was opened for black men, both as soldiers and officers. There were many racist prejudices against them, but black troops fought with heroism and courage in some battles of the Civil War.