The book starts off by stating,
Time series econometrics is concerned with the estimation of difference equations containing stochastic components.
Hence the book naturally begins with a full-fledged chapter on difference equations.
Difference Equations
A few examples of difference equations are given such as Random walk model, Structural equation, Reduced form equation, Error correction model to show the reader that difference equations are everywhere in econometrics.Any time series model indeed is trying to explain a univariate variable or a multivariate vector in terms of lagged values, lagged differences, exogenous variables, seasonality variables etc.