The set of manipulators let you change the value of a dataref directly with a mouse-click - the various flavors control how the dataref is changed.
These manipulators are the natural corollary to the command manipulator, which runs a command on a click.
Why have both? Commands are good, but they don't cover 100% of sim functionality (just like datarefs don't cover 100% of sim functionality). By having both, it will be possible to control switches and buttons that are best accessed by a dataref change.
For the basics on commands vs. datarefs, see here.
The generic instruments already write to both commands (via the trigger instrument) and datarefs (via the rotary instrument) - these new manipulators provide the same functionality for 3-d cockpits.