Square roots of a complex number

My book says that given the complex number $z$ with modulus $r$, its square roots are $±√re^{iΘ/2}$ where $Θ$ is the principal value of $\arg z$. My question is that why must we consider the principal value of its argument?

Answer

We consider the principal value by convention. But also if we take the other value $\frac{\theta}{2}+\pi$ we find the same two roots because $e^{i(\frac{\theta}{2}+\pi)}=-e^{i(\frac{\theta}{2})}$