What is a straight razor primarily used for?

A straight razor is primarily designed for shaving facial hair closely. The blade of a straight razor is long, flat, and typically very sharp, which allows for a close and precise shave. When used properly, it can remove hair at the skin level, providing a very smooth finish. This is especially beneficial for those who desire a clean-shaven look without the irritation that can sometimes occur with other shaving methods.

The other options, while they may involve the use of a straight razor to some extent, do not capture its primary function. Cutting hair can refer to a variety of tools used in barbering, trimming mustaches might be done with clippers or scissors, and shaping the beard could involve various techniques and tools other than a straight razor. However, the essence of the straight razor's design and intended purpose centers around achieving that very close shave on facial hair.