A bra (brassiere) is designed primarily to support the breasts during normal daily activities, or during exercise (a “sports bra”). It can also serve to make breasts look larger, fuller, or create cleavage (a “push-up” bra), or to make them look smaller (a “minimizing bra” or “minimizer”).
Typically a woman would need one or more of these functions, since, generally speaking, it is women and not men who have breasts. But while that is generally the case, it is not universally true. Occasionally there are some men who have some degree of breast development.
If a man does have some degree of breast growth, then he might have a need for support (especially when exercising) or one of the other functions a bra performs. In particular, it isn’t hard to imagine that such a man might have use for a minimizing bra.
Since a bra is a garment specifically designed for the breasts, then any person who has breasts might have a reason to wear one, even if that person is a man.