In the end, Steam provides vital services for game development companies. Distribution, marketing tools, content delivery, and a purchasing system. With Steam, a developer never has to worry about having to shut down its content systems or purchase system due to lack of funds or any other reason. Steam is one of if not the biggest reason for allowing independent video game companies to get their content to the public. Some game companies restrict their games to Steam as a way to protect against piracy. We will not be using this method. Instead, we will rely on the Citizen's Registry to authenticate users playing online. Of course, you could still technically play the game offline or on servers with no authentication if you had the files.
For the user, Steam is a centralised system for purchasing games and providing social infrastructure through their overlay, friends list, voice call, achievements, etc. A game released on Steam is reviewed by Valve first, the process requires the developer to provide sufficient documentation and the Steam testing team to go through the game to ensure it meets their standards. Developers and publishers are forced to comply with Valve's strict content policies, meaning that the customer only has to rely on the reputation of Valve/Steam when purchasing (to a certain extent). A customer who purchases a game on Steam knows they have a certain amount of security. Purchasing from an external vendor that might have no reputation does not provide the same security. Steam also provides a rating system, allowing customers to voice their opinions. A potential buyer will not have to worry about the developer/publisher modifying the ratings to suit them.
Supporting Steam and other digital distribution platforms is positive for the indie game community, it's popularity has allowed games like ours to get our product on a distribution platform (without a publisher).
You don't have to use Steam if you don't want to, but know that some games simply could not exist without it.