When starting a construction project, the place to begin is at the bottom of the building. Once a good foundation and footers have been laid, a grid is generally constructed to build the bottom and upper floors. No matter how high a building will become, each floor must have structural members securely in place before the next floor can be built. This ensures the upper floors will not collapse during construction.
Large buildings are often used as office space. Companies differ in size, and may not need an entire floor to house their offices. While all the structural components are in place and cannot be moved, the office configuration is often up to the tenant. They rent the square footage they need and then have the space configured to fit. Additions such as office walls are not structural and can be placed where the tenant wants them.