The instant speed creature and ability to counter a spell gives you a two-for-one punch
For many playtest cards, you'll need to make a generous assumption that basic game rules would be updated to allow them to work
While Paradigm Shift is by no means the primary way most decks using Jace and/or the Oracle empty their library, it is a way to do it and thus it has seen play in Legacy and Vintage, where the Oracle especially is a very common win condition
**Note: Foil cards get an extra layer of ink applied to them before the CMYK layers called the underprinting layer
Outside of a few very specific cards (which will be mentioned first) that require deck sizes above the minimum, all the other reasons you might want to are more fitting to casual magic: Battle of Wits - A win condition that specifically requires you to have more than 3 times the minimum deck size left in your deck to win means starting with about 4 times that minimum, if not more
Well, its hard for two-color cards to be high up on my list, as having two colors will naturally reduce the number of decks where its playable