Stupid question from an FNG -

I have seen quite a few MB diesels with the EGR removed and a blocker plate installed.
Or, I see a small cartridge air cleaner installed.

I am curious about the how and why. This forum seems to be the best place to get traction on issues like this, so I figured I'd give it a whirl. Last year I test drove a gorgeous Rod-Bender whose owner had performed the surgery. He was quite happy with it. I'll ask Mister Google as well and read up.

Many thanks in advance to all replies.