Question about exhaust system/flex pipe?

[Teh Luckinator]

I was looking at my whole exhaust system, and I found something weird:



A while ago I looked it up somewhere, and it's... just there. It's not the cat, but some big ass hollow space. I'm just wondering, is it really needed? I really want to put in a flex pipe, because the exhaust system shakes like shit (it broke off the place where the cat and the exhaust join together... wonderful), but I'm wondering if that hollow part is needed. If it's for emissions, then I'm keeping it on for sure, but yeah. I'm just wondering what it does. And is there any other place I could install a flex pipe? I'm getting a new cat back exhaust system, so yeah.