Registered mail is handled with a higher level of security than standard mail, including the use of a special lock box, signatures at every transit point, and a traceable routing number.
It is basically considered an add-on and does cost extra. I personally have never sent anything Registered but I have received things Registered. I know I was able to track it the whole entire time that it was enroute. Thus why I think alot of folks abroad choose to use it when sending to the US. I do know however it is pricey when using it to send out of the US. It has a starting rate of $10.95 plus the cost of whatever shipping method is chosen prior.
I hope that helps somewhat. Hopefully someone from abroad can enlighten us a little more on why it is used so much when sending to the US.