Steam maintains:

• Server costs for the storefront, fast game file distribution and the variety of other services they offer to developers (eg the friends system, game invites, the Workshop, the marketplace, cloud saves, etc...).
• Steam handles payments for you, worldwide. This alone is already a pretty huge undertaking that many underestimate.
• Steam does a ton of analytics for devs.
• DRM.
• Support staff for handling customer complaints, refund requests, etc...
• Sets up seasonal sales and handles promotion for devs willing to participate.
• Steam allows you to resell Steam keys on other platforms, which forgoes the 30% cut entirely (creating new keys is free), as long as you won't resell the keys cheaper than the game's price on the Steam storefront itself.
• At high enough sales, Steam cuts back on the part they take.

All of this stuff is free and bonus for devs. You pay a one-time fee to set up, which you get back if your game sells enough copies (it's a really low bar, unless the game is total gunk you're going to reach that threshold). Not to mention the costs involved in bringing your game to other devices it wasn't originally built for, allowing you to gain access to a larger share of the market (again, for free. It's even available to users using a competitor).