We've seen some of the versatility of ANCOVA in Chapter 9 and Chapter 10. In application, it's often used in ANOVA settings to adjust or "control for" a covariate that may be masking real treatment differences. In regression settings, researchers may be focused on a family of regression relationships, and are interested in testing for significant differences among regression coefficients across different groups.
These are like two sides of the same coin: in terms of model development, ANOVA and Regression approaches converge in the general linear model as ANCOVA. Mastery of ANCOVA methodology is arguably one of the most important tools to have in an applied statistician's toolbox.