Remote access (such as via a phone or computer) and automation. It is nice to be able to do anything you want in your home. For example:

• Attaching a humidity sensor to a smart switch for the exhaust fan in your bathroom to turn on and off automatically based on humidity level.
• Have lights change hue based on time of day to assist with your circadian rhythm.
• Shut off all lights and TVs in the house at 3am.
• Using a water detector in the laundry room or kitchen to inform you of a water leak by sending a notification to your phone and beeping loudly.

I personally would not want toilet flushing as a smart feature.