Scenes and automations solve two different problems, and the names are precise: a scene is something you trigger, an automation is something that triggers itself.
A scene is a saved group of device states — "Movie Night" might dim the lights, lower the curtains and lock the front door, all from one tap. It always waits for you to ask for it; nothing happens until you press the button.
An automation runs on its own, based on a schedule, sunrise/sunset, or a device changing state — "lock up at 6 PM every day" or "turn on the porch light at sunset" needs no daily input from you once it's set up.
In practice the two work together: an automation's action is very often just "activate this scene" — so you get the one-tap convenience of a scene on the days you want to trigger it yourself, and the same scene running automatically on the days you'd otherwise forget.
A good rule of thumb: if you find yourself doing the same sequence of taps at the same time most days, that's an automation waiting to happen.
