Kohler kitchen faucet base plate
contacted Kohler by email and got a prompt reponse as below…
The plastic under-cover should be installed on the faucet. This plastic piece, when installed with the plumbers putty will eliminate the gap you are seeing around the base of the faucet, and keep water from leaking under the faucet. Water under the faucet could lead to damage under your sink and potentially cause damage to the faucet. It is highly recommended that this part be installed with your faucet if you are using the escutcheon plate intended for 3 hole installations.