Because the oil will pool in the line and will be excessive if not allowed to drain back. The can will still do its job only better because it will more easily remove the vapor and allow it to condense into the bottom of the can.
I think that the vacuum will pull the dirty oil/vapor into the catch can,and not collect in the line.
The oil being extracted by the PCV valve is not clean oil.It's more of a contaminated by-product of blow by exhaust gas,which every engine produces,some may be producing more than others.
I dont know about you,but I dont want that dirty oil going back into my engine.
Then,with a air oil seperator/catch can plumbed in between the PCV and the vacuum source,the dirty oil/vapor collects in the catch can until it can be drained out,thus keeping it from being recycled back into the combustion process.
This is assuming that you have a baffle or strainer where the PCV valve goes into the valve cover to prevent engine oil being sucked out.
At least,this is my take on how this should all work.
Guy