It illustrates the trend of usage while still conveying the association between traffic volume and date. Primarily it dramatizes the comparison between days. Whatever the theory exec row likes it because it is easy on their gray matter. If the numbers are stored in a database then you'll be able to refresh the graphs based on the period of data placed in the database. For example, if the data is queried from a nightly summary stage out of a data warehouse then you'll be able to group data by day.
Lets look at the HTML to display a single bar in the above graph to see how it is done.
This tells the browser how high to make that table. Use a dark color so you can superimpose white lettering on top for a nice effect. Line 10 is the subtitle for this bar. It is in the second row of the outer table in the same column as its bar. To add bars you add columns in the outer most table filled with inside tables holding the bars of varying heights.