Link functions allow the systematic component (β0+β1X1+…βpXp) to live on (−∞,∞) while keeping the μi consistent with the range of the response variable. For example, g−1(η)=exp(η) ensures that the mean in Poisson regression models, λi, is greater than or equal to 0. Similarly, g−1(η)=exp(η)1+exp(η) ensures the mean of logistic regression models, pi, is between 0 and 1 (Figure 14.2).
I found this chunk a little unintuitive...I'm wondering if it might be better to phrase it something like:
Link functions $g(\cdot)$ transform the response onto the unbounded scale of the linear predictor $\nu_i = X_i \cdot \beta$, allowing regression coefficients to operate without constraint. Inverse link functions $g^{-1}(\cdot)$ map the linear predictor back onto the scale of the response. This ensures that predictions respect their distributional constraints (e.g., non-negative for count data, bounded between 0 and 1 for binary data).