These come with all the same cards as the normal decks, except with a surge foil treatment for all cards and tokens
With Annie Joins Up in play, you can actually use a similar loop to create infinite mana via Relic of Legends (casting Honest Rutstein will cause Ronas untap trigger to happen twice, so you can end up floating an extra mana each time with Relic of Legends)
Special sets are smaller than standard sets but they often have a stronger theme and narrative pull
Graded Population Report: Mismagius #10 Visualize the distrbution of grades in the chart
Examples of such dual sets include Blue Shock [XY8] / Red Flash [XY8], To Have Seen the Battle Rainbow [SM3H] / Darkness that Consumes Light [SM3N], and Snow Hazard [SV2P] / Clay Burst [SV2D]
There are a few reasons why it's such a good card, so let me explain them to you conversationally