When designing something new, the only way to really know if it works is to test it. For various reasons, when designing buildings, the testing tends to occur after the building has been built; during the course of its use as a workplace.
The results can be seen by noticing poor placement or layout of structures in relation to the tasks being done, having to make changes to the building so work is easier, retrofitting structures to match equipment or tasks, the occurrence of incidents and near misses, discomforts or injuries appearing among the staff, and generally feeling like it could have been designed better.
Ergonomics can help by providing evidence-based information to start a design. But another important aspect that ergonomics provides is knowing and accepting that variation is present through a wide variety of factors, which means that testing is required to evaluate the theory.
All workplaces are different, weather they use the same equipment, or do the same type of work, there is variation in environment, people, management style, workloads etc. that can all have an impact on how a workplace is designed.
By performing mock-up evaluations, the designer and client are given the opportunity to test the design, and fine tune it before the big money is spent on building it. This avoids more big money being spent to change it, and the countless other related issues.