Why present data with pie chart?

Because it shows how much each element of a whole unit takes up. So if you had totals for each of the 12 months of a year, like sales totals, a pie chart would show 12 slices, one for each month. You could then have an indication as to how much each month made up of the total sales for the year. You can get the pie chart to show the actual value of each slice of show the percentage each slice represents of the whole pie.