Video games, like religion, have those who know its rules, and those who are uninformed. Similar to religions, it wishes to convert those who do not understand it, into loyal followers. The idea of going door to door passing the "good news" can be shared by both (I submit exhibit A).
Casual games (note I am not talking about Social Games, which is currently a pejorative) are a leading off point to hardcore games. They start as simple, approachable games, where the rules are easy, or even intuitive. They take a simple concept, like clicking on an object to highlight it, and instill a codified gaming idea into the minds of its users (note that they are not necessarily gamers at this point).
Step by step a casual game leads the player into deeper interactions. Depending on the game, it might only go into the shallow waters, wading on the surf. At the end, it gives the player a reason to seek a similar game. That game, may be as casual, or may be designed with the idea that the person played a game like it before. Suddenly, new concepts enter the language of the user, and step by step, game by game, the user becomes a gamer.
For so long, we have had A for okay, and B for cancel, that even the X-box follows this convention (and further expand on the concept by making A green for go, and B red for stop). The Playstation, realized how ingrained button layouts were for gamers, that they made the O and X buttons to match the locations of the A and B buttons on the Super Nintendo controller. These are interface mechanics that codify into a players mind.
These general gaming ideas, are designed to make it simpler for a gamer to master interactions with the game. This effort, to make hardcore controls easier to grasp is for attracting newer players, but can also aid those gamers who have not yet mastered a game's interactions.
Tony Ventrice over at Gamasutra says that there are 6 things that makes a game hardcore, and it is those 6 things that need to have incremental steps in order to draw the casual gamer into the hardcore space.