This answer to Nature of “the silky black 'highly flame-resistant felt'” on Block 5 Falcon 9's, and its intended functions? contains the following:
This came into Falcon 9 design with Block 5 improvements. As Elon Musk mentioned in updates about Falcon 9 Block 5 it is carbon fiber hydrophobic thermal protection developed at SpaceX. The reasons why they replaced are pretty obvious. One reason is the heat factor and the other reason is expenses.
Question: Why would the Falcon 9 Block 5 carbon fiber thermal protection material need to be hydrophobic? Is this for its thermal properties, or purely aesthetic (stain-resistant?) or perhaps is this just an quirk of the way the material behaves and isn't for anything in particular? Might keeping it clean keeps the thermal emissivity high, or the visible reflectivity low (i.e. "looks cool")?