Why is Macbeth assigned?

Macbeth is the shortest of the four classic Shakespeare tragedies, and its fast pace and compelling atmosphere make it very accessible to modern audiences.
But it's also assigned for the same reason all Shakespeare plays are assigned: it continues to resonate as a portrait of human desires and relationships. The character of Macbeth—before we meet him in the play—is clearly an honorable warrior, acclaimed by many, including the king he serves. But his ambition, stoked by the Weird Sisters and even more by his wife, kindles his evil side, to which he succumbs.
Then we witness the process of disintegration as he commits more and more crimes, becoming "o'er shoes in blood" and turning himself into a monster. This is far from unique to Macbeth. We witness similar horrifying journeys constantly in modern life, which is another reason the play continues to be taught and performed.