In practice, it's often good to have comments that explain what the code is doing, even if the code itself is quite understandable. It can be a lot quicker to read and understand a one-sentence English description than it can be to read 10+ lines of good code. If the code in question generally has accurate comments of this nature, then working with this code becomes far easier.
And then there are always those cases where rather poor code was written in the first place, it lacked comments, and then a maintenance programmer needs to figure out what it does in order to make some changes to it. Real-world constraints prevent the code from being completely rewritten, but adding comments explaining what it does can be invaluable for anyone else who will ever need to deal with the code.