A heat map comes handy when you want to compare a metric collectively with two dimensions.
The multiple colour intensities are used to represent the intensity of metric for the corresponding dimension values. Scale adjacent to the chart helps understand the relation between the metric colour and it's value.
They use color to communicate relationships between 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 for a month.