Shaving removes more than hair...When you shave, as well as removing stubble, the razor bladesalso remove some thin layers of old and dead skin from what is called the stratum corneum, which is the top part of skin you can see.
This is not a particularly bad thing as our skin is designed to continually shed itself to reveal newer skin underneath, so in time it recovers it’s thickness (although this process slows down as we age).
However, the multiple layers of the stratum corneum (see the illustration showing the anatomy of the epidermis) are there to provide a protective barrier for the skin against UV rays from the sun, toxins, pollutants and bacteria, as well as helping the skin retain moisture so it stays soft and flexible, not dry and flaky.
When the thickness of the stratum corneum is reduced, its efficacy as a protective barrier is also temporarily reduced, making the skin much more vulnerable to UV damage, pollutants and bacteria and less able to retain moisture.
By using a moisturiser or balm with an SPF after you have shaved, you provide the extra protection your skin is missing whilst it naturally starts its recovery process. The SPF provides the UV protection and the moisturiser or balm provides a temporary occlusive (impenetrable) layer on the skin that helps the skin retain its moisture (so helping prevent it from becoming dry, flaky or itchy) and it also acts as an important barrier against bacteria and pollutants.
Also applying a moisturiser or balm post shave is the best way to ensure the actives in the formulation can penetrate the skin, as the removal of the old and dead skin allows these active ingredients to penetrate the dermis (deeper in the skin under the stratum corneum) where they can add the most value.