Contact sensors are most often used as part of smart security systems, put on doors and windows to tell if they've been opened when they shouldn't have. But they're also really useful for knowing if anything has been opened or closed - blinds, fridges, drawers, garage doors, doggie doors, medicine cabinets, you name it. If it opens and closes you can stick a contact sensor on it.

You can use them as standalone devices, or pair them with a smart home system like SmartThings, Apple HomeKit or Amazon Alexa to do useful stuff such as have lights turn on, your alarm system disable, and maybe the TV turn on or the kettle start boiling when you open the front door.