The urge to collect all the items, achievements or secrets in a game.
"A bit. Oh, hello, quest cape, max cape, master quest cape, master max cape, completionist cape, trimmed completionist cape..."
"Clearing achievements also gives a reward (coin,gem,item,weapon skin,and also Play Games XP) further encouraging players to play more to unlock all of them"
"Clearing achievements also unlocks items, characters, and upgrades, further encouraging players to play more to unlock all of them."
"Unlocking every achievement is necessary in order to view the game's true ending."
"achievements for collecting certain numbers of fish cans (1 is in every level) and postcards (3 total in game)"
"The more achievements you complete, the more stickers you get which can unlock new costumes for customers. Some achievements are unlocked just by progressing in the game so you do have to continue playing to get them but you aren't forced/urged to complete any achievements to advance the game."