There are tons of extremely powerful effects that become even better when doubled up, like Portal to Phyrexia's ability to force your opponents to sacrifice three creatures when it comes into play, now forcing all your opponents to sacrifice six creatures instead
Count down your life total with the included Click Wheel and store your collection in the sturdy card storage box, including dividers to keep things organized
For every chance they might spin an opponent into a god hand, theres every chance they just give them a bunch of land and throw a finely sculpted hand into the bin
The card has some synergies with blinking and returning cards to your hand, and its a good target for web-slinging as well
March of the World Ooze doesn't create ooze tokens, but rather creates big oozes out of your other creatures, though it is still good with Doubling Season
I like doing this instantly to respond to targeted removal like an opposing Vindicate, Chaos Warp or Swords to Plowshares coming my stuff's way, or just to dig for my own answer, make triggers and loads more