My Son-Inlaw called me over as he was replacing a two bowl sink with a garbage disposal to a Single Bowl Sink with a disposal and had a question. Attached is a picture of the plumbing. The lower pipe received waste from the two sink bowls, the other higher and to the right received waste from the dishwasher. To the left is the waste pipe and vent. Any thoughts why two drains, one higher than the other? Thanks!