The low blow can be the most devastating punch a boxer takes during a fight. Whether they are deliberate or unintentional, the damage is debilitating, and the result of the fight can be determined from it.
They were once legal in certain combat sports, but regulations and rules have been applied to keep the one who is daring enough to go low from doing it. If they do, they are usually warned or have a point take away.
Very rarely are they disqualified, but as you'll see in these videos, that's not always the case.