Not sure what Roblox is in first place. An integrated game engine and hosting site?
Once you write a game or create a world, you can click and publish it for anyone to join.
But you can jump from game to game quickly.
Many of the games are similar-ish to popular games but the barrier to entry is low, share assets, games are easy to jump in and out of together with your friends.
My niece was about 300 dollars into this game before my sister got involved. She was using 'oh get me itunes gift cards' from relatives so mom would not notice. She was using all of the tools of an addicted person to use that game. Her sister on the other hand got bored with it and gave up on it.
I was sorely disappointed - I'm as perplexed as you are how they became so successful. It is indeed a total mess.