Custom-written software is software that is written specifically by and for an organization, company or person as opposed to buying already written software. While most organizations use mass-produced purchased software, some organizations choose to write their own software to cater to their specific needs.
Most software that is used today by companies and organizations comes from specific software companies that sell their product for commercial use. Programs like Windows and Microsoft Office are mass-produced softwares that are for sale. Some companies, however, do not wish to buy software or may not be able to find software that is for sale to meet their needs. These companies will then use custom-written software that is specific to them that only they can use.
For an organization to write its own software, it needs to have access to a computer engineer and somebody who knows how to write code. Writing software is an extremely involved process that takes a lot of time and resources. The benefit of using custom-written software is that it will work exactly the way the organization wants it to. The organization gets to dictate the layout and functions of the software themselves. Also, custom-written software helps keep sensitive information within the company, as information on purchased software could potentially be obtained by an unwanted third party.