This example is explained at the post https://geoexamples.com/other/2019/02/08/cog-tutorial.html
The represented data is a temperature interpolation over Catalonia, and the GeoTIFF is very big. In this case, we only use a small region of the data. Since the GeoTIFF is cloud optimized, only the nearest blocks to the data are downloaded, and the amount of data and used bandwidth is small.