72% of top-100 US games by Revenue this year has the collectibles mechanics. In February 2017 there were only 21% of projects with such mechanics.
GameRefinery tells that collectible mechanics is a great way to increase user retention and monetize them more efficiently. Plus, many people just like to collect things, which changes the user experience with a game for the better.
In the full version you can find the best examples of collectible mechanics: sticker album from Board Kings; weapon gallery from Pixel Gun 3D; heroes collection from Marvel Strike Force and many more with a description of how the collectible mechanics implemented in very each project.