Linked from
The 11 pages that link to Degrees of freedom, each with the reason it gives.
Confidence intervalRelated: They affect critical values in intervals that use estimated variance.
Student's t-testRelated: They determine the t-distribution's shape and often depend on sample size.
Sum of squaresRelated: Variance estimates divide a sum of squares by degrees of freedom.
Chi-squared testRelated: They select the chi-squared distribution used to calculate the p-value.