Editing is an art and a science
Why does an editor make changes?
The science of editing
I make changes for various reasons.
Sometimes I make changes for typographic reasons – for instance, to lengthen previous paragraphs so there is not just one line at the bottom of the page (a widow) or to shorten the text so there is not just one line at the top of the next page (an orphan).
Sometimes it is to change text into plain language. For instance, I remove unnecessary adverbs (words ending in -ly that describe verbs). Often, I remove unnecessary clause connectors (like that).
The art of editing
Grammar, rhythm, assonance and alliteration
Sometimes it is to correct grammar, sometimes to improve rhythm, sometimes to create assonance (same sounds) or alliteration (same first letters).
I should be able to justify every change I make, if the client queries it. It is their work, not mine.