Games can take a long time to make. From conception, to a playable something and all the way through to the finished product can take years, sometimes even longer. Gamers end up watching a product through this process, from start all the way to the often grizzly finish, and wishing that they could get involved somehow. Steam Early Access has given them a window into this process. What it does is allow games to be sold earlier in their lives, often for a lower price, and gets people into the process earlier.
It has allowed some games to develop brilliantly; gamers ideas are implimented, and feedback is gathered by the developers and the game changes over time. Early Access has, however, been exploited by people looking to make money quickly for their projects. They charge full price for a game that is extremely early in development, charging what they might do for the finished article.