Documentation can be as simple as a README.md file in your projects route, docblocks on top of each class or a complex wiki detailing how the project functions.
It also tends to be an afterthought or never completed, while a development team deals with other priorities.
Why does it matter?
Documentations value is a subtle one and doesn't really show until you have a new hire or someone leaves the company.
It's crucial for onboarding new people to a project and can help reduce time when integrating new featured or resolving issues.
It's much easier to work out how a function works when it's general intent is detailed nearby.
Simply put - time and workflow.
Your internal development team, while familiar with the codebase, will have valuable development time taken away from them when they have to write complex documentation.
It can also be quite costly to hire someone whose job it is to only write documentation for your company if you don't have enough projects to justify this expense, the alternative is your developers.
Throughout my career, I've always documented my code and projects, which has helped me to no end and allowed me to accomplish far more than without it.
Hiring me to look at your projects and generate documentation at regular intervals will help speed up your development process and prepare for any unfortunate circumstances.
It happens and it's can be fixed.
Depending upon the complexity of your project, it might be appropriate to outsource and save you money!