Employees at an end-to-end product development firm have gained additional perspectives into the user-centered design process from team members in other functional areas. This often informs their own processes for product development, and enables them to keep key considerations from other departments in mind as they themselves iterate on your project. This is especially true when the firm is a small- to medium-sized business because there is more likely to be intermingling of employees, rather than siloed departments of functional service areas.
When a business chooses to work with a full-service product development firm, the individuals they are hiring for their project are integrated with people in different functional service areas on a day-to-day basis. Their colleagues and internal team members for other projects span different roles in the product development process. Because of this, they are used to working and collaborating with people who have different viewpoints than themselves. Working with people who understand different stakeholder values can make for a more pleasant overall experience and can produce better results for the user and client.