There are probably countless reasons for businesses to outsource writing tasks. Here are the ones that seem most compelling to me.
1. Time: If you don't have the time to prepare the words for your new website or corporate brochure, that's a likely reason for outsourcing the job.
2. Resources: If you don't have the resources in house, or if your writing resource is taken up with other projects, that's a good reason to call a specialist.
3. Independence: Sometimes, when you have been working on a project for a long time, you might be too close to it. You might feel that on that project you can't write dispassionately. This does happen. And it's a sign of maturity to be able to recognise the situation and outsource the writing.
4. Professionalism: You could write it yourself, but you feel that you are a generalist and you want a specialist writer. You choose to outsource, for the job is important and you want it done professionally so that it reflects well on your organisation.
5. Efficiency: Because professional writers, write every day, you can presume they write efficiently. Writing is their work so they just get on with it. By contrast, in many offices, writing jobs just keep getting delayed. (That's probably why there are so many out-dated websites and brochures that feature tired and outdated text).
6. Quality: A sixth reason for outsourcing writing jobs is to get the job done properly. You call on your Accounting Department when you have an accounting issue and your IT specialist when you have an IT question. Similarly, when you have a writing challenge, one reason for outsourcing is to get the job done properly.
Maybe there are other good reasons for outsourcing your writing challenges. If you want to mention one, please feel free to do so below.