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For emission regs. the crankcase oil fumes are normally fed back into the induction system to be burnt during combustion. A catch can system re-routes these fumes into the 'can' which then needs to be drained regularly or may have a drain back to sump pipe.
As you can imagine, feeding the fumes into the induction system effectively contaminates the mixture with the effect of lowering the octane rating.
Does it help to reduce emissions from the exhaust?
Ha,ha - good try! In theory, yes but in the closed circuit system, unless the blowby past the pistons was really bad, it would be virtually unmeasurable. Don't forget, the catch can is vented so crankcase fumes (but not the oil mist) go to atmosphere.