What's the deal with Malignance?

I'm no Xerath main or 1-trick, but I definitely enjoy me some Xerath from time to time. If I take long breaks from League and come back and want to instantly climb back to Emerald, I just play Xerath mid a bunch. I used to run Comet, but currently this is what I've been doing (taking some inspiration from a couple threads I've seen on here):
- Dark Harvest with Cut Down and Legend Haste in secondary tree
- Sorc boots ofc
- Luden's (more burst, gets into DH range quicker)
- Shadowflame
- Rabadon's or Void
- Void or Rabadon's (whichever wasn't bought already)
- Horizon Focus or Zhonya's usually

I do see people going Malignance on him quite a bit. I think that Korean Xerath goes Malignance into Stormsurge into Shadowflame or something like that? I guess I don't really understand the motivation behind it and why people would prefer it over Luden's for stronger poke. Are we ulting off cooldown for something (like DH stacks maybe)? Are we just so good at our ults that we're using it to 100-0 squishies off cooldown? Is it only good into certain situations?

I'd appreciate any advice you guys have on this, or how you make decisions on runes or items as well! Or if you have any advice on landing ults. Sometimes my ults are very good, and sometimes it feels like a plat player is scripting the way I can't even land 1 little ult spurt. Thank you in advance!