Although one needs "horizontal space" to build something on, knowledge about the lay of the land is indispensible if you want to build a road, railway, sewer, water pipeline, or buildings, where knowledge of the gradients and estimates of volumes of excavations are needed.
It's also a great advantage if the heights are all relative to one known point; usually mean sea level, rather than a series of dispersed benchmarks that might not agree with each other.
When the Canberra-Queanbeyan area was selected as the site of the national capital and a competition was set up to choose a design, the planners needed a topographical map.
This was provided by detail and contour surveys done during 1910-13. As more and more data became available, height information was progressively improved and integrated.
Former Australian Survey Office surveyor Peter Daly gives a potted history of level datums in the ACT (PDF 1.3MB) and some of the processes of integrating data from different eras.