The only juicer I have any experience with is a Champion. Sold it years ago, wish now I'd kept it. Nowadays there are several quality brands with similar mechanisms. Basically, they slowly masticate the vegetables so as not to heat them, then press them through a screen, so the juice comes out the bottom and the fiber comes out the end.

Nothing wrong with throwing it all in the blender, but then all the fiber stays in and it can be hard to digest if you drink a lot of it.