Q:
Why use an external Technical Author?

A:
If you don't have dedicated staff producing your literature it is more difficult to produce consistent quality documentation when you need it. 

When you engage an external Author, their time is spent wholly on your documentation project and not attending to other duties.

An external Author brings with him/her a wealth of industry experience and knowledge, the quality of your documentation will reflect this.

You can engage a Technical Author when you need one, this is especially useful if new projects arise that would otherwise over commit your existing team.