This is useful in a few situations:

A customer removes an item, but the structure of the site does not allow calling of removeCartItem. This would lead to a situation where the removed item still exists in Rejoiner's database. If the entirety of the current cart's data is available (this is usually the case), you can call clearCartData to remove everything and then make the corresponding setCartData and setCartItem calls to add it back.
A customer removes everything from their cart and you do not want this customer to be treated as abandoning a cart, but would rather have them be treated as abandoning a browse session.

This endpoint does not require any params.

_rejoiner.push(['clearCartData']);