From The Heroes of Might and Magic III wiki

The altar's function depends on your hero's alignment.

At the Altar, if you are Good, you can sacrifice only artifacts; if you are Evil, you can sacrifice only creatures; and if you are Neutral, you can sacrifice both.

Your hero receives experience points in return.

Comment: Normally you would not sacrifice troops but usually there are artifacts you do not need so Evil aligned heroes are at a disadvantage. A hero's alignment does not change if bought in a differently aligned town. Buying an Evil hero in a Good or Neutral town does not help.