Copywriter
Writes the copy that decides whether anyone contacts you. A good one asks many questions about your business before writing the first sentence, because there is nowhere else to get specifics.
When your project needs this person
On every site. This is the role most often skipped and most strongly felt.
What happens when they are missing
Copy that could sit on any competitor’s site. The visitor cannot tell why you rather than someone else, and leaves to compare on price, because price is all they can see.
How this appears in an offer
Should be a separate line with a page count. If it is missing, the copy is your job.
What they do NOT do
Not a proofreader and not a translator. All three are different services at different prices.