Show the relationship between two or more variables. Be mindful that, unless you tell them otherwise, many readers will assume the relationships you show them to be causal (i.e., one causes the other).
Smith, Alan. How Charts Work (p. 83). Pearson Education. Kindle Edition.
Correlation allows us to see the extent to which two things are related. Visualising correlations is important because it helps us see the extent to which things are connected.