Technology

All the extra's it brings;

• Valve supports Proton which allows any game in you library to be played with on Linux or even on Mac if your device can run it
• Common redistributables which automatically get installed for the player if needed
• Notification system
• in game micro transaction handling through Steam
• Multiplayer API's you can hook into (like matchmaking)
• A complete stat and achievement system
• Chat and friends system
• Steam cloud for save games
• Steam input, so controllers just work for your game (even Epic will point you to this)
• Digital Rights Management (DRM)
• Error reporting
• HTML API so you can show HTML pages in your game
• Persistent inventory system
• Key system so you can give them away, or sell them on other platforms
• Leader boards
• Remote play (couch co-op over the internet)
• Voice chat
• Steam Workshop
• Valve anti-cheat (VAC)
• Steam VR layer that allows any vr device, even has a controller remapper build in
• A customer support team that will actually help you, the customer, instead of throwing roadblocks left, right and center
• The store has a gift option, Epic does not (as far as I know), so you can buy a game for someone else and gift it to them via Steam

Epic has:

• A store, that has popups and pulls straight back to the store which is very annoying
• Multiplayer (EOS), which is free for anyone to use
• Unreal Engine
• A CEO that actively shits on things Valve related and in the past has shared his dislike for Linux

Epic has had plenty of time to copy the success of Steam by just using their blueprint, but they won't switch to being customer friendly, so instead Tim Sweeny (Epic CEO) is just hating on them (Valve). The other launchers are just terrible to begin with, the EA launcher has been buggy from the start and really unstable, Uplay is equally as bad. The only real competitor is GOG in my opinion, but they are small in comparison to Steam.