|Editor at work|
See a definition of the term editing services in the section "Editing Services" on this page:
Editing firms may employ a team of in-house editors, rely on a network of individual contractors or both. ... Such firms are able to handle editing in a wide range of topics and genres, depending on the skills of individual editors. The services provided by these editors may be varied and can include proofreading, copy editing, online editing, developmental editing, editing for search engine optimization (SEO), etc.
I do understand that some self-employed editors may have to work with editing services for a while to build up their experience, and that some editors prefer to work through services rather than on their own. But I cannot believe that the authors who obtain editing through editing services are getting top-quality work for the low fees they pay. Cheaper is not always better.
Many of these services don't allow direct contact with authors, which makes it harder for editors to do good work: Authors are the ones who know what they're trying to say in their manuscript, so it works best when the editors working on the manuscripts can have conversations with the authors about problematic passages.
Take a look at this article in the journal Nature for more on how editing services work in the academic world: "The Manuscript-Editing Marketplace." The article talks about AJE, Edanz, Editage, and MacMillan Science Communication, the latter of which is owned by Nature's parent company. It compares how those companies work with how the online editorial marketplace Peerwith works.
I looked at this page today (April 3) of Peerwith's website and saw very low rates that some Peerwith editors charge their clients: US$400 for a 10,000-word manuscript on Asian studies, US$220 for a 4500-word manuscript on medical mycology, and US$300 for a 4700-word manuscript on molecular biology. On top of that, Peerwith charges editors a service fee of 10% to 20%. (Find the information on services fees by clicking the "Fees & Payments" button on this page.)
Peerwith's structure, like that of editing services, does not seem to be designed for editors to earn livable incomes. Editors who want to work through such an intermediary will have to work on a lot of manuscripts in a very short period to earn much money at all, which can mean they must do lower-quality editing. That's a losing proposition for editors and authors alike.
Full disclosure: I do not work with editing services. Instead, I work directly with physician-authors who are non-native English speakers. In 2016, I took part in a panel presentation at the annual meeting of the Council of Science Editors. For that session, I spoke about being a sole proprietor who works directly with authors, and AJE's quality manager spoke about self-employed editors who accept project assignments through AJE.