If I understand correctly this goes to think of spin as a quantum measurement already, but there are spin measurements with a certain result (for example a
$\hat{\sigma}_z$ on an electron with spin status
$\left| \frac{1}{2}, \pm \frac{1}{2}\right>$. Furthermore, it does not explain why it behaves exactly like an angular momentum from any point of view (especially the addition of angular moments)
