To allow vendors to implement them and developers to use the appropriate implementation. For example:

-moz-border-radius-bottomright:1em;

-webkit-border-bottom-right-radius:1em;

What would be the alternative? You couldn't only do border-radius-bottomright: 1em, because that wouldn't work in webkit. Nor would you want to force the vendors to check with each other first before implementing features, because that would slow down progress and make it slightly more difficult for new open source browsers to get started.