Air Oil Separators (AOS) or Catch Can's are an essential component that should be fitted to your performance Engine.
In stock configuration, the un-baffled crankcase pressure is re-circulated back into the intake stream. This can be harmful for two reasons:
1) Oil vapor mixed with the intake air effectively lowers the overall octane rating of your fuel. The more oil vapor that is sucked back into the engine, the more likely you are to experience engine knock (also referred to as detonation) that can destroy an engine.
2) Oil vapors cause oil & carbon build up over time. The intercooler, intake manifold & intake valves suffer the worst. This oil vapor can create air turbulence that leads to various running issues.