Fifth Dawn is best known for introducing the Sunburst mechanic, rewarding players for using multiple colors of mana, as well as further expanding the artifact-heavy environment that defines the Mirrodin setting
Thanks to this, they wont break the bank either, coming in at just over the $10 mark for a basic copy
The other use I can think of is that Karn's card draw isn't worded as "drawing," so if you're a horrible person and want to lose your friends, you can pair Karn with mean cards like Possessed Portal or Omen Machine to lock them out of the game while you keep your hand nice and full
Ratcatcher is also an important payoff for getting the best rats out of your deck quickly
If you don't control any creatures with power 4 or greater when the ability resolves, the ability won't do anything
If you have graveyard recursion, youll often want to chump block with your Chaplain to get it into the graveyard so you can get it back if you dont have a way to sacrifice it