An internal document distributed to Apple Authorized Service Providers confirms that the silicone membrane included on the keyboards of new 2018 MacBook Pro models is to “prevent debris from entering the butterfly mechanism.”
The document, obtained by MacRumors, is taken from Canadian and European versions of Apple’s internal 2018 MacBook Pro Service Readiness Guide:
Keyboard and Keycaps
The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism. The procedure for the space bar replacement has also changed from the previous model. Repair documentation and service videos will be available when keycap parts begin shipping.
Although the U.S. version of the Service Readiness Guide doesn’t mention the membrane, it does link to a separate internal document that does include info about the membrane.
“Butterfly Mechanism Keycap Replacement MacBook Pro (2018)” document:
Caution: The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism. Be careful not to tear the membrane. A torn membrane will result in a top case replacement.
The membrane was first discovered last week, during a teardown by iFixit.