Yes, multiplayer is a good point. Roblox is an easy way to make games that are
1) Multiplayer (skipping all netcode, account systems, and so forth)
2) Have a secured, safe, and trusted way to make transactions
If you were to try and make a multiplayer, microtransaction based game in Unity, that is a large amount of work, especially for a younger developer, and it's _critical_. Messing up payment code has huge consequences.
Making reliable multiplayer games is tough: I'd love to see a serious API service that does the same thing for "real" game engine/libraries!