I am learning étale cohomology, and we defined the étale cohomological dimension of a scheme $X$ as the minimum of $n$ where $H_{ét} ^n (X,F)$ vanishes for all the torsion sheaves $F$, yet I don't get why we only restrict our view to torsion sheaves.
This seems to be a very stupid question, but I really have no idea why we only talk about torsion sheaves as while discuss étale cohomological dimensions.