1) Just when you think you are the greatest author ever, they let you know that you are not.
2) Just when you think you are not a good writer at all, they let you that you are.
3) In other words, they keep you grounded.
4) They are great content editors.
5) They are great line editors.
6) They are great sources of information.
7) They are a pillar of support, and they expect the same from you.
8) They can be your best literary friends.
These are just a few examples. What others can you list?