Disclaimer: This article contains custom card designs. If you are a Wizards of the Coast employee you have my full permission to use any designs or ideas within this article without my explicit written consent or prior knowledge, or without giving me any sort of credit. I love this game and I want to help it in any way I can.
So Magic has these things called core sets, yeah?
A yearly catalogue of baseline spells and flavor, primarily intended to be a entry point for new players and a repository of useful effects for tournament level play.
Since their introduction into Magic, Planeswalkers always appear in a one-per-color-cycle in each core set.
Starting in Magic 2011, R&D started one of my favorite new traditions; Planeswalker signature spells.
The purpose of these effects was to introduce players, especially newer players or players who don't have the resources necessary to get a hold of a lot of product and might not ever see a Planeswalker, to said characters. After all they are significant cards and the main characters of the story, its a good idea that people invested in your product know who's important.
Someone just starting Magic isn't going to know who Liliana is, much less, why they should care. But when they start seeing her name show up on a lot of cards, they're going to grasp fairly quickly that she's someone important.
Here's the explanation for their design and concepting.
Now that's all well and good, except that there's a problem.
A big one.
Sorin and Gideon never got uncommons!!!
We can't have that now, can we...