They put you up on mana with very little investment
For example, if your commander has the ability to gain a lot of life, you might build a Commander deck with cards that benefit from maintaining as high a life total as possible, and focus on out-enduring your foes - or you might use your life total as a resource, burning through it to fuel powerful abilities and relying on your commander to keep yourself from succumbing
The appeal on this one is going to be more for Magic diehards than anyone else, so it's not going to be essential for every planar deck, but it's going to elicit plenty of reactions when you land on it
In the mirror or against GB midrange, setting up Karn behind a mass of tokens or after Settle the Wreckage attacks opponents on an angle they arent really prepared for
Not counting your opponents graveyard is certainly a downside, but costing half the mana is a big enough upside to make up for this