Board Thread:Questions and Answers/@comment-27364860-20151215024649/@comment-24577221-20151215050550

The benefit is that the happier a monster is, the more quickly it generates coins. A 100% happy monster generates coins twice as quickly as a 0% happy monster. Happiness doesn't affect how much money a monster can hold; that's determined by the monster type and the monster's level. Whatever the monster's maximum is, it will reach that limit twice as quickly when it's 100% happy than when it's 0% happy.

In some cases, that can make a big difference. If you're collecting coins from your monsters before they reach their limit, you can get a lot more coins when they're happy. But some monsters reach their limit very quickly even when they're 0% happy. A Potbelly reaches its limit in only 6 minutes or so when it's 0% happy, and about 3 minutes when it's 100% happy. If you're not collecting coins from a Potbelly more often than every 6 minutes, you don't get any benefit from making it happy.