In general, writer means someone who writes anything for a large audience.
When used on this site, writer refers to what I do, which is write technical manuals and other technical material. This includes step-by-step “how to” material, background information, technical marketing material etc.
Writer is often a synonym for author. This is not really the case when you work with me. Yes, in many ways I am the author of the material. More correctly though, we, meaning experts at your company and myself, are co-authors. I do most of the work, but it is your company name that appear on the document as the implied author.
I write to best represent your company and the products you have sold.