1. Galaxy lets you download offline installers.

2. Steam (usually) introduces its own DRM that forces you to sign-in to install and launch the first time, even from a backup. Galaxy has no such requirement. Games today are designed to work around a client install method, so you can (usually) copy and past the files and have a DRM free copy when the client isn't required. However the offline installers are safer than this for a variety of reasons.

You can launch the games you install with Galaxy directly by there .exe files so if you don't want to start Galaxy you don't have to.

Offline installers contain galaxy components, the installer is setup like galaxy. There isn’t much difference nowadays.
You do need galaxy for most features, like roll back, achievements, and for some games like cyberpunk to get all the content, and for most multiplayer. It’s optional other than that, mostly.
Oh, and you can only buy epic games from galaxy, might still have epic drm in.

The EGS launcher begs to differ. ;-)

StingingVelvet and wolfsite covered all of it. You don't need to log into Galaxy to launch your games. It's entirely optional. When my ISP went down a few weeks ago, all my games were able to launch and work fine offline. No cloud saving or achievements syncing with Galaxy until my connection was back up. So nothing to worry about there.

Besides somegames requiring Galaxy for optional cosmetic stuff, all your games will work without it. And besides bugs and lack of features, the only other relevant concerns with Galaxy involve telemetry, which is covered in this thread here. Though to what extent GOG does with this collected data outside of improving Galaxy is unclear.