Speculative offices offer important efficiencies. Not only do they make it easier for a company to move into a space and begin work immediately, but they also help when it comes time to relocate.
When an office is left empty, it can take months or even years before another tenant steps in and fills the space. This means that your company's employees may be waiting around while their former office sits empty.
When you choose speculative office space, however, you don't have to wait on someone else's schedule: if there are other companies interested in moving into the same building as yours at some point in the future (and if those other companies happen to need exactly what you're looking for), then they will likely sign an agreement—which means that any delays caused by tenants having trouble finding new places would no longer affect your business operations.