Reference: McDuff-Salamon's book J-holomorphic curves and Symplectic Topology, second edition.

In section 6.7, the book introduces the moduli space of $\{J_z\}$-holomorphic curves (see page 184 for example), where $\{ J_z \}_{z\in \Sigma}$ is a smooth family of almost complex structure parametrized by the point $z$ of the domain $\Sigma$. What could be advantages of this compared to the ordinary moduli space for an almost complex structure $J$ independent of $z$?

In particular, when this book discusses the gluing theorem in chapter 10, we also consider parametrized almost complex structures. How does such a slightly general gluing theorem apply more widely than the gluing theorem only for a fixed almost complex structure $J$? I am just wondering if it is really necessary to consider the most general cases at the cost of simplicity.