They are used by browser developers to implement CSS3 properties in development builds before they are confident of meeting the CSS3 spec and/or achieving a reliable bug-free render.
Gradually, each vendor introduces support for the spec property (sans prefix) and, later, eventually removes support for the vendor prefix. This is a gradual process and varies between vendors and rendering engines.
But to implement CSS3 properties reliably, efficiently (only the necessary code), and with maximum compatibility is very easy. If, at any time, you cant recall which browsers and engines support each CSS3 property, use prefixes, or have no support, just check out css3please.com. That always shows the status quo.
Don't sweat the absence of some style elements in some browsers: that's part of graceful degradation, or "do what we can to support the IE community while doing what we really want for everyone else..." Just make 100% certain that no part of your CSS fails in a way that hurts usability, legibility, or accessibility. OK?
(I am amazed, though, that no-one else at this forum has told you this already.)
Or just settle for graceful degradation and the reality of browsers.