Well, again jmho, but headgear allow the opponent to use full force. That brings training closer to actual application, without quite the same risks. But, here's from a place that sells headgear:
http://www.boxingdepot.com/headgear.htmlStay Pretty!
Headgear does not prevent concussions well. You still need to keep your hands up. You can get knocked out while wearing headgear.
Boxing headgear does protect from cuts, scrapes, reduces swelling and prevents skin damage. Headgear also does a great job of preventing cauliflower ear. Headguards with cheek protection, protect a larger area.
Personally, I didn't like headgear because they made my head a bigger target. They also cut down visibility, or at least my perception of it. However, I would never advise someone to train without headgear; I would advise him to avoid sparring with head-hunters until he can "defend himself at all times."
"A man is rich when he has time and freewill. How he chooses to invest both will determine the return on his investment."