A butt, or head butt, is an intentional use of the head to inflict damage on your opponent. An accidental butt lacks the intentional part that equation. In other words, the head butt is an accident.
So who cares about an occasional butt? Well, head butts both accidental and intentional are the causes of a lot of stopped fights due to the cuts that get opened up because of them.
Your noggin is hard and if you slam it against a nice soft fleshy eye, crushing the tissue above your eye against the eye socket, a deep gash can be opened up instantly.
Then you not only have an accidental butt, but you also get to see what is referred to as a bleeder. Where an intentional head butt is illegal, a butt that is ruled accidental can actually win you the fight. Just ask Holyfield about it in his fight against Hasim Rahman. In that fight, an accidental butt by Holyfield led to a referee stoppage forcing them to go to the scorecards and low and behold Holyfield was in front. Who says using your head doesn't work?