It's mostly done in cases where you'd play a very heavy AP focused seup to 1 tap people, usually paired with Rabbadon's Deathcap. With Staff and your E pen you have like 50% magic pen total. It's extrmely niche, and ever since Bloodletter's got added I don't think I've had more than like 4 games with either Void Staff or Cryptbloom, but it gives a lot of instant damage. If you Q a squishy with 80 mr, just from Staff and pen boots that target would have under 30 magic resist for you, and because you don't need to stack it like Bloodletter's it means the full pen is instantly applied from the very first damage they take. Other than that, you never really want to run it. Before you could do it if the enemy team had 3+ super magic resist stackers or if your team was heavy on magic damage so you needed to deal with heavy mr from the enemy team, but because Bloodletter's exists now it basically means you never need to do it into a tank as you can stack up Bloodletter's very easily vs them and it even has very good value for your magic damage dealing allies.

I do not use an in game item set, so there's no link for that. I just have the items placed on the guide with the explanations and some builds for different cases.

Glad to hear it. I'm happy to help any way I can!