I think the necessity and importance of physical descriptions of characters depends on what's being written. If, for example, you're writing fantasy or science fiction and you have characters that have physical attributes that a normal human does not possess, obviously you need to describe them. I think it's important also if you're writing erotica or if physical traits are important to the character - like someone who is a love interest for your protagonist for example. You also need to consider how you're telling the story. A first person narrator isn't likely to go into great detail about someone they already know but might be more detailed when first meeting someone. Whereas if you're writing in third person, you can provide as much or as little detail as you like with less risk of it seeming awkward or out of place.