You see this all the time with marine antennas, but you’ll also see it with some TV and cellular antennas designed for home use. It’s not just to keep the antenna pretty, there’s actually a good reason.
In the case of marine antennas, fiberglass is usually the covering of choice. It’s light, durable and transparent to RF signal. For TV antennas it’s just white plastic, which is less durable and more subject to corrosion. But, then again the sea air isn’t usually as much of a problem on land.
No matter what the material, the purpose is to keep the items inside from corroding. Antennas are commonly made of aluminum because it performs pretty much the same whether it’s corroded or not. However, some antennas may have copper elements or a mix of copper, aluminum and steel. Copper is more sensitive when it’s not corroded but almost useless when it is. That’s why copper wire is always encased in some sort of sheath.
Adding that layer of plastic keeps whatever’s inside corrosion-free and dry. It’s common sense for harsh environments.
There’s also a little bit of vanity involved, at least with marine antennas. It’s common to see a bright white leisure craft and that shows it’s clean and well maintained. A white fiberglass sheath over an antenna matches the nice white look of the boat.