What is a straight razor primarily used for in barbering?

A straight razor is primarily used in barbering for shaving and outlining facial hair. This tool is designed to provide a close, precise shave, which is ideal for creating clean lines and defining the edges of facial hair styles, such as beards and mustaches. The sharp edge of the straight razor allows barbers to achieve a smooth finish and detail work that other tools may not be able to manage as effectively.

In contrast to the other choices, shaping eyebrows typically requires a different tool, such as tweezers or small scissors, to achieve the desired precision. Layering hair involves cutting techniques that are best suited for shears or clippers, rather than a straight razor. Trimming split ends usually requires shears specifically designed for cutting hair without affecting the overall length. Thus, when considering the main functions of a straight razor in a barbering context, it is clearly focused on the art of shaving and the meticulous outlining of facial hair.