You always get a land, even if youre ahead on land, which is a nice upside compared to other catch-up creatures
Especially if theyre just energy, perhaps, because you can run them in a lot of decks
Example: Diseased Vermin reads, in part, "At the beginning of your upkeep, Diseased Vermin deals X damage to target opponent previously dealt damage by it, where X is the number of infection counters on it." If Diseased Vermin phases out, it "remembers" how many counters it has and also which opponents it has previously damaged
Double-Faced Modal Lands and Their Implications Zendikar Rising preview season is now afoot
Common 8
The second ramps a Plains (any) as well and while it costs twice as much mana it does not require a player to have more to ramp up, so that will always work