The ARCH model is: $$\left\{ \begin{align*}& X_t=\sigma_t Z_t, \ \{Z_t\} \sim IIDN(0,1) \\ & \sigma_t ^2 =\alpha _0 +\alpha _1X_{t-1}^2+\ldots+\alpha _p X_{t-p}^2 \end{align*} \right. $$ After fitting such a model we can forecast $\sigma_t^2$, but (I think) the process $\{ X_t \}$ is of interest, not $\{ \sigma_t ^2 \}$. So, actually why do we fit (G)ARCH model, since $Z_t$ has expectation 0 hence the best forecast for $X_t$ will be always 0?
Maybe we fit (G)ARCH model because we always fit it along with, for example, an ARMA model (so called ARMA-GARCH model)?