The main reason for using an existing Architecture Framework such as TOGAF is that an existing framework adheres to what actually works in a real life scenario. Using an existing framework will simplify and expedite any new architecture development and allow for future organisation growth.
Another reason for using an existing framework is that a framework such as TOGAF contains an existing set of resources such as templates, patterns and other artifacts that are immediately available for use.
Finally, by it’s nature, the design of Architecture is a complex and extremely technical process. TOGAF is able to provide a strong guideline about what works and what does not which enables organisations to create open systems based solutions that will meet the immediate and growing needs of the business.