I have A LOT of automations. Some of my favorites are:
Automatically set "house state" (morning, day, evening, night, not home, etc.") based on alarm state, time of day, last movement, etc - this state controls a lot of other automations
Push message to my phone if outside camera detect a person when house state is "night" or "not home" (using Frigate for object detection)
Automatically turn off lights at night or when not home. Automatically set lights to right level when home
Push message when window has been open for more than 20 minutes.
Push message if windows or doors are open when leaving the house or at night
Push message with "morning update" and "evening update". Including today's weather, if I should sell some stocks as price has reached threshold, etc.
Basically, I no longer has to remember to turn on of off lights and if I forget something important in the house (such as an open window or turning on the alarm), it reminds me to do so (if it can't do it itself)