Editors tend the forest and the trees of writing. They need not only the ability to see the big picture and a reliable path to conclusion, but also the ability to provide care and attention to the smallest pieces--to every word and piece of punctuation.
A firm grasp of language and grammar usage and rules is essential for providing guidance and navigation. Without it, an editor will only persist in doing harm when they should be providing service. Knowing the rules is vital for helping writers make the best decisions about their writing, but also for knowing when and where the landscape of the author is best left wild and true.
My editing portfolio contains an expanded editing philosophy statement, and examples of proofreading, light copyediting, medium copyediting, queries, and memos.