Lets face it, not every writer is created the same nor is every freelance writer who is looking for freelance writing a good English speaking writer. Sure, there can be someone who is said to be an English speaking writer but when it comes down to it English is their second language which can result in some rather funny or interesting faux pas when attempting a description of something which they have only read about in their native language. So an English speaking writer has some advantages over a non-English speaking writer.
Some advantages of an English speaking writer:
Hiring an English speaking writer will not only get you a freelance writer who understands most of the concepts of what you need written about but they will also be able to convey their thoughts so they are understood by people who read your blog or website or your forum. An English speaking writer with English as their first language understands the nuances of the English language and there are many of those for sure. Many of the UK sites request a writer who speaks English to also live in the UK so that they know what house removals or zebra crossings are. In the US they want the writer to understand moving companies and crosswalks.
Using the English language literately:
For example if you hire an English speaking writer and you want to sell elevators or impart information about elevators then you want them to know what an elevator is whereas an English writer who lived in the UK may not understand the concept because they understand lifts instead. In the US lifts may be fork lifts or shoe lift inserts not an actual elevator. Many people can speak the English language but when it comes to writing in it they find the concepts as abstract and cant convert what is abstract into fully understandable literate sentences like an English speaking writer is able to.
The written word:
Just because a person speaks well it does not translate that they also can write well. Writing is difficult. Getting the grammar correct is much more obvious in the written word then is noticeable when speaking. When speaking you dont have to choose between a colon and a semi-colon. Also the written word is more formal and mistakes are not well tolerated like they are in the spoken word. So with the written word and therefore written articles the expectations are much higher. This means using someone who is a native English speaker becomes more important for articulate articles.
Writing is difficult:
Just about everyone can speak in English if they live in an English speaking country but writing is more difficult even for a potential English speaking writer. Even an English speaking writer who is not a good writer or cant put their verbal concepts into a more formal written article will not be an effective writer. Writing is more difficult and finding a good English speaking writer becomes much more important when looking for freelance writers.