Argument condition in Barnes Integral Representation

Question

We know that the Barnes Integral representation is given by $\frac{\Gamma(a)\Gamma(b)}{\Gamma(c)} _2F_1 (a,b,c;z)=\frac{1}{2\pi i} \int_{-i\infty}^{i \infty} \frac{\Gamma(a+s)\Gamma(b+s)\Gamma(-s)}{\Gamma(c+s)} (-z)^s ds$, where $|arg(-z)|<\pi$. I want to know what role does the argument condition play in convergence of the integral? I can understand that $-z$ is being taken in place of $z$ due to the presence of $-z$ in the integral. Can someone please explain?