It catches oil vapors that make it past the PCV valve. The actual "NEED" for it is debatable. I put one on my 5.7l "Just because". There are threads on here debating if you really need it. If you are thinking you will ever "boost" your car it wouldn't hurt to have on on hand.Thanks! That smile hits everytime I do a remote cold start. This is my first car I have ever purchased on raw emotion. Not at all practical for where I live or what I "need" from a vehicle, but damn I love it so far. What is the purpose of the catch can anyway? Is the PCV system not enough? Thanks for your time.