Like with most instances of outsourcing, it generally comes down to available resources. If you’re a company of a fairly large size then it makes sense to hire an internal copy writer/content writer, but if you’re still in your growing stages (or just planning to remain a small business), then outsourcing these requirements can make a lot more financial sense. While the temptation could be there for you to do it yourself, this should be resisted.
It can actually be pretty damaging for your business’s brand if you have anything less than professionally written copy. This is because people who are looking at your promotional material are going to be forming an opinion based upon a very limited amount of information, and for many, misspellings, mistakes with grammar, and a poor flow with the text can be very off-putting, even if it is just subconsciously.
What a copywriter offers a business isn’t just writing with the correct grammar though, so if you’ve got that area of writing down already, this still doesn’t mean that there isn’t value in hiring a copywriter. They should also be able to match the tone you need to provide the most benefit to your business, something which generally requires training.