Reluctancy to abandon a partially completed goal, even one forced upon the player.
"you get different aquarium decorations for being there on set anniversaries or taking part in seasonal events, and get different icons depending on the level you are at"
"There are achievements for various feats as well as completing and 100%ing the game on each difficulty."
"It gives 'gods' after completing series of challenges grouped in episodes, but these are kinda just puzzles you have to do in order to access / progress through the main campaign"
"There's achievements, that's pretty much it. I don't think achievements are *that* bad though, it gives you more to do, but are completely optional. The achievements do have rewards associated with them though, so maybe it can become an obligation."
"There are some achievements for completing certain objectives such as collecting or completing different difficulties."
"Game rewards anything for hatching dragons with certain type combinations to rewards for defeating a certain ammount of dragons with a dragon of a certain rarity."