Why the keel is necessary for the installation?

When we install the flooring, firstly the keels are fixed in the ground surface with the T-clip and then we put the solid decking board on the keels. You know the wpc decking board can't be put on the ground surface directly. For one thing, it can affect of the planeness of the flooring surface after installation. For the other thing, it is not good to control the drainage of the bottom, because the wpc decking board is mainly used for outdoor, which rains a lot. So keels must be used to hold the solid decking board when installed.