Pie charts are popular because they are easy to create and understand. The main use of a pie chart is to show comparisons between different categories. They are widely used in the media and in business reports as they give the reader a quick idea of the proportional distribution of data.
They are only effective when you're comparing 3-5 different data points with different quantities that are easy to differentiate. Pie charts are one of the most used and hated charts. Its shape is very familiar, but this visual representation is easily misleading if not used properly.
When the number of data categories is small (less than 6) and you don't need to show progression over time, a pie chart might be appropriate. Use a pie chart if you want to illustrate the dominance of one category over the others.