The intended usage of a construction project not its exterior appearance primarily determines its architectural style. I completely agree with this sentiment as the internal requirements which can differ drastically from building to building will determine important aspects of design that would need to be accommodated for it to serve its purpose.
If a residential building is being designed the proper utilisation of space inside the building is of primary importance as it will be used by people of all ages and would need to be equipped to provide a comfortable living standard for its inhabitants like have courtyards for the children to play, allow for adequate sunlight in the house, have cross ventilation in all areas these essentials will impact the design far more than the external facade.
Buildings that are designed for special purposes have to be cognisant about the requirements of the intended user for example while building a hospital the overall design will need to accommodate for safety and regulatory requirements of the health care industry, bio hazard storage solutions, round the clock people flow, the modifications required to allow stretchers and wheelchairs access and many more complex needs.
In conclusion, while the external look of the building is an important aspect as it should be appealing so as to encourage it use but the most important aspect of designing a construction project should the designated usage of the it.