They were made to support the magecraft mechanic, so cards with magecraft and prowess are a good fit
There are two big reasons to run these cards: Firstly, 5 manas the going rate for an extra turn with no strings attached
However, I want to acknowledge the following rule for the Commander format: Rule 13: Abilities which refer to other cards owned outside the game do not function in Commander without prior agreement on their scope from the playgroup. Karn is a really good card for any 60-card format because you are allowed a 15-card sideboard from which you can pick cards that are "outside the game." However, in Commander, there are no sideboards or wishboards
Thats half your Deck, right there
He also owns a PSA 10 First Edition Charizard which he purchased from Gary Haase for $150,000 (116,129 / 138,021) in 2020
I could sit down with any of these for a couple of evenings and polish them off some of them I already have, months after leaving them unfinished but it always feels like a chore