Players like finding things like coins because of the dopamine it gives. Number go up, the genre, is popular for this very reason. Seeing a stat raise higher and higher makes people's brain activate in a way that just gives them a shallow, but satisfying, sense of accomplishment. To make it even more satisfying we would need to add things like achievements for unlocking the coins, or something to spend the coins on. These give the coins a real purpose, making it so collectors can boast about their coins, or so players can use their coins on something to benefit them.