Because - as it was already stated - there are OC procedures you simply need to know, prior to CCR training.Why not?
We all agree that CCR may (stressed - MAY) be more risky than OC. So being already comfortable uw, with expierience achieved while diving OC, limits the risks.
I hugged my HH.I am going to go home now and hug my Meg.
BTW, it reminded me I need to change the sensors before I will end up in the water again (it's still winter here, so way too cold).