Complete the Collection

The urge to collect all the items, achievements or secrets in a game.

The desire to collect things is a strong urge that game developers often exploit to get you to keep playing a game with the hope that one day you'll find all the rare items and have a complete set.

For people who collect things, the collection can have significant emotional value. It may connect them to something that they enjoy or feel strongly about. Some people collect for the thrill of the hunt, others may collect because it gives them a sense of security or control. Others may derive great satisfaction from having a complete set and feel anxious when they are missing a piece of a set.

All of these psychological effects contribute to making you keep playing the game, even if the "playing" of the game is not enjoyable anymore. If game developers are constantly releasing new content, then the players are always needing to "catch up" and may feel like they need to spend money to do so.

Sometimes reaching the 100% completion rating in a game can take hundreds of hours of additional tedious work beyond the normal scope of the game. Sometimes, some of the collectable items are only available via special events, and because players may have a fear of missing out on those items, they will continue playing and paying, even if the game is no longer fun.
Citations and SourcesWikipedia :: Psychology of Collecting


Stick War: Legacy Stick War: Legacy
"Again, there's achievements for 100% on each difficulty."

Play Together Play Together
"You get rewarded for completing a set of fish or bugs that you have caught"

Daily Dadish Daily Dadish
"There's like 15 playablee character, each needs 10 stars to unlock, which means 10 levels, and there's only one level a day. You cannot buy stars and there's no gameplay benefits to the characters, though. Just skins."

Toca Lab: Plants Toca Lab: Plants
"Get the plants"

Pocket God™ Pocket God™
"There's no reward for collecting all of the 'gods' but it helps with the campaign alot more"

"There are 20 "Shiny Trinkets" scattered across the map to collect, and you get in-game music awards for getting more of them as well as unlocking a secret area with a unique minigame."

Cookie Run: Kingdom Cookie Run: Kingdom

Dragon RPG: Dragon Village M Dragon RPG: Dragon Village M
"It's the whole point of the game"

Severed Severed
"Some areas of the game give you rewards for coming back later with abilities you didn’t have the first time. The game doesn’t have fast travel and enemies don’t respawn, so these optional collectibles require you to mindlessly backtrack for a relatively long time"

Stardew Valley Stardew Valley
"In more recent updates there are special items and quests you can only access by increasing your 'Perfection' rating aka moving closer to 100% complete."