Color can hide a lot of small mistakes in a portrait – a slightly busy background reads as “vibrant” instead of distracting, and a so-so expression gets rescued by a nice tone in the light. Black and white removes that safety net.
Without color to lean on, contrast, expression and composition have to carry the whole image. I shoot for black and white when a face has strong texture or emotion I want the viewer to sit with, rather than get distracted by what someone is wearing or where they are standing.
In practice that means paying closer attention to shadow shape on the face and making sure the background actually separates from the subject in tone, not just in color. It is a slower way to shoot, but it is a good check on whether a portrait works on its own merits.