I looooooove Minecraft! The iOS version hasn't got all the features of the PC version but I think they constantly update it to try and catch up with it.
The appeal changes depending on what a person likes doing, some people play it to try and get to The End and fight the boss, some like exploring to find a perfect home location, but I think most people like trying to create elaborate structures out of the blocks.
The thing that is huge though is modded Minecraft, people are constantly modding the game to include new items, new features and new gameplay. Some people create custom maps for people to download turning Minecraft into a puzzle game, RPG or adventure. There are servers for people to play multiplayer, sometimes just doing vanilla stuff but sometimes there are servers dedicated to a game made in Minecraft or MMORPGS and more
I could go on and on lol I think to children it's like Lego but you have to collect the resources - there's a sense of accomplishment when you complete something that other sandbox games can leave you missing