Linked from
The 20 pages that link to Covariance, each with the reason it gives.
VarianceRelated: Covariance determines how dependence affects the variance of sums.
Linear regressionRelated: Covariance helps determine fitted slopes and coefficient uncertainty.
Karl PearsonRelated: Covariance supplies the numerator of the Pearson correlation coefficient.
Statistical learningRelated: Correlated feature changes can reveal which properties belong together.