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)