Why do you need to normalize choropleth maps?

What does it mean to normalize data? When and why do you need to do it? This video shares an example in ArcGIS Online and ArcGIS Pro. 

The data used in this video is Census 2021 Dissemination Areas and Buildings from Halifax Open Data, and North America Hexbins 10km: Sahr K, White D, Kimerling AJ. 2003. Geodesic discrete global grid systems. Cartography and Geographic Information Science 30(2):121-134.