Most virtual tabletops are general-purpose. They have to be — they support hundreds of systems. The cost of that flexibility is setup: you configure the thing before you can play.
AoI Tabletop Online only has to know one game, so it can do the admin for you. It handles the mechanics, the tracking and the bookkeeping, and leaves the GM free to actually run the story.
Two views, one table
The GM view carries the map, initiative and everything the players should not see yet. The player view carries the shared map, the tokens and the character sheet. Nobody has to learn a piece of software to sit down at the table.
Free for players, forever
Because the software is part of the game rather than a separate product, there is no reason to charge a table full of people for the privilege of being invited. The GM subscribes; the table plays.