Checking the effect an operator has on a state

[whatisreality]

I've been reading these notes: http://www.vcpc.univie.ac.at/~ian/hotlist/qc/talks/bloch-sphere-rotations.pdf
And on slide 11, to check what the rotation operator is doing, the state is pre- and post-multiplied by the operator so that the calculation performed is
$\rho' = R\rho R^{\dagger}$
Why do we need to do that and not just
$\rho' = R\rho$?
Thanks for any help! :)

 

[phoenix95]

In the same link, take a look at how ρ is defined(slide #5) and how it transforms under the action of a Unitary operator(slide #8). Considering R being Unitary, the answer to your question should be straightforward.