From the resource:
We love complete and nicely formatted data. It means we spend less time restructuring and poking at a sparse dataset and more quickly get to the visualization, analysis, and insights. It’s why I like to work with Census data so much. A lot of time and research is put into making sure the data is as complete as it can be.
But a lot of the time (most of the time?), the data you work with is not complete. There is missing data. Available values can be sparse across time and space the farther out you stretch.
Read the full resource here.