Registered mail is sent secured, it goes from one secure area to another along the route. Each time it is moved it is accounted for and easily traced/tracked. It does not mengle with the regular mail. It is usually much slower because of all the additional accounting it must go through.

If it is priority also it simply goes first ahead of the other registered mail- but it still has to go through all the stops and accounting slowing it back down.

Registered mail seldom is lost. Also it is the only 'legal' way I can send currency through the mail.