Besides tapping for both red and green mana, cards that have both land types gain the extra benefit of being more easily searched for by fetch lands, spells, and abilities
Fluctuator allows for free cycling, which may not seem like a big deal now, but in a Standard full of cyclers, it was pretty nuts in Astral Rift
The glimpses were brief, and the features were seemingly animated, however
In addition, most of the spells in this set that trigger them tend to be great, including winners like: Thunder Magic Ice Magic Dreams of Laguna (triggers with flashback) Retrieve the Esper Eject Sorceress's Schemes Choco-Comet UR gets to play a ton of good interaction and card advantage, all of which can set up cascading advantages with its value creatures
Priest is only $1-$2 right now, which is a small price to pay for a potentially swingy rare