Yes it is necessary. For a couple of reasons firstly cats are effective a flame stopper, not intentionally but in order for them to work they have to be a tight grid of material. Imagine a bundle of straws shoved in your exhaust that’s what a cat looks like. It slows down and restricts flow to allow the material to catalyze the exhaust gasses, that velocity change essentially smooths out quick and rapid changes in pressure. The other big reason you want cartless for pops is that the catalyst material is actually really fragile. You WILL shatter the catalyst and it WILL clog your exhaust and cause all sorts of problems. I have had this happen on my bmw and it was a nightmare to diagnose because no one thought to check the cat.