In merge games, players usually need to merge two lower-level objects into higher-level objects. In contrast, there are merged match-3 games, where three lower-level objects are merged into one higher-level object.
A typical example is Merge Dogs. For example, two pug dogs can be merged into one Shiba Inu.
The main driving force of merged games is usually a sense of completion: players combine objects to unlock upgrades. Taking full advantage of this, rewarding players for the time they spend, can maintain their participation in the game.