Automated builds are a pretty important tool in a game developer's toolbox. If you're only testing your Unreal-based game in the editor (even in standalone mode), you're in for a rude awakening when new bugs pop up in a shipping build that you've never encountered before. You also don't want to manually package your game from the editor every time you want to test said shipping build, or to distribute it to your testers (or Steam for that matter).
You've got this great idea for a competitive multiplayer game. Maybe you've even built a prototype and tested the core game loop extensively and found the fun. You think maybe you could just throw that hot potato up on Steam Early Access and make a buck or three. After all, good gameplay is ultimately what determines a game's success, right?