A Heat Map widget comes handy when you want to compare a metric collectively with two dimensions.
The multiple colour intensities are used to represent the value of metric for the corresponding dimension values (darker color denotes greater metric value) . Scale adjacent to the chart helps understand the relation between the metric colour and its value.
In Heat Map widget, colors are used to communicate different data values that would be would be much harder to understand if presented numerically in a spreadsheet.
For example the above graph eases the qualitative analysis of total Impressions by helping you get a comparison of variation through days of week across four months.