The original purpose of vendor prefixes was to give vendors the ability to add their own non-standard features for use in their CSS implementations. However, most of them are used for experimental versions of things that eventually become standards.
If an experimental property is unprefixed, it implies that it's the correct, standard version of a property. If every browser renders it differently, then there isn't much of a standard. Instead, a browser avoids implementing an unprefixed property until it's sure that it's done so according to the standard, then it starts supporting the unprefixed property as a way of saying it supports this particular standard correctly.
Not every feature has a prefix; indeed, a vendor creates a prefix only if it deems it necessary.
