For 35mm film photography most photographers preferred an 85mm lens for 2 reasons. First, it allows a longer working distance. It keeps you outside the subject's 'comfort zone' which creates a more relaxed atmosphere. Longer working distances also yield greater depth of field at a given f/stop. Second, also a result of the longer working distance, it prevents subject distortion. If one part of the subject, say a hand, is much closer to the lens than another part, say the face, the hand will look much larger than the face. Less noticeable, but still very real, is the distortion caused by photographing a face close up with a short lens; the nose will appear oversized in relation to the ears.
Allowing for a 1.5 crop factor, you would need to use a 57mm lens to get the same reult in digital as an 85mm.
Many photographers prefer an even longer lens for 35mm portraits. I personally like to use a 105mm Nikkor. I've seen Monte Zucker shoot a portrait using a 500mm on his Hasselblad. Yes. you read that right, 500mm. It gave a very long working distance. Using long lenses like that gives you extremely good control over your backgrounds.