Purpose of ID in Keys

I am looking at the docs here and had a few questions.

There is a section explaining ID as below
ID (used to identify key within an account)

and then in the section below there is an example of weight:
Key ID: 1, Weight: 500 Key ID: 2, Weight: 500 Key ID: 3, Weight: 500

I had few questions related to it:

  • If there are multiple keys is weight required?
  • What if I create multiple keys with different ID but same public key?
  • Is there any reason for introducing ID, if public key serves the purpose; ID seems redundant if weight is not specified/required or used.