To expand on Pheryll's answer, in the beginning, helping someone always required visiting their city and figuring out what building to help. Then after you helped a neighbour you had to search around in their verge for chests, the same way you do for truffles (event currency/keys/snowflakes/candies). There were lots of them, but they weren't really worth looking for. Then they reduced it to three, and made the contents better (at least for people with the Crystal Library/Bell Spire)
When they released mobile, they weren't making you load everyone's city to help them, so they made the chests automatically open when one was generated by random. Then they replicated that on browser. They're still chests, but you never see one unless you've already opened it, so it's not intuitive.