In portrait photography, you get better facial perspective if you stand back a little from the subject. This means you need to use a slightly longer focal length than usual to fill the frame, and an 85mm lens is normally considered the ideal focal length (or 85mm equivalent on a camera with a sensor smaller than full frame). Longer focal lengths can work too, but you quickly run out of space in normal interiors.
Besides, the 85mm focal length also lends itself to wide apertures without excessive weight or costs. Using a wide aperture will throw the background out of focus and highlight attention on your subject. Portrait lenses typically have a maximum aperture of f/1.8, f/1.4 or even f/1.2.
This is one reason why ‘portrait’ lenses are fixed focal length ‘prime‘ lenses rather than zooms – it’s not possible to make zoom lenses with a maximum aperture that wide.