What is the main benefit of adding a serum to hair?

Adding a serum to hair primarily provides the benefit of adding shine and reducing frizz. Hair serums are formulated with silicones and various other ingredients that coat the hair shaft, which helps to smooth down the cuticle layer. This application results in increased light reflection off the hair, thereby enhancing its shine. Furthermore, by smoothing the cuticle, serums can prevent moisture loss and combat humidity, which are the main contributors to frizz.

While some serums may contain ingredients that give a slight volume or texture boost, their primary function is often geared towards enhancing shine and maintaining a sleek appearance. They also do not specifically stimulate hair growth; that function falls more to treatments and specialized products focused on the scalp and hair follicle health. Similarly, heat protection is usually a feature of specific styling products intended to shield hair from thermal damage, rather than the core role of serums. Therefore, the correct answer emphasizes the benefits of shine and frizz control provided by hair serums.