You moved into a home with a pre-existing home security system
Sometimes when you purchase a house, a modern security system is already installed. All the components work, are placed in the correct places, and are ready to go. In this example, you might discover that you have to call and start a new contract. A Vivint installer will still show up to ensure there are no integration issues, and they could offer options on where you should replace bad outdated components. It's a preliminary appointment, and you can have your alarm monitoring set up without having a bill for any new alarms and cameras -- unless you get some new devices.
The previous owners can transfer an existing Vivint contract to you for a one-time set-up fee to make the transfer a snap. If the owners downsize to a condo or retirement community, you keeping the security contract might be a condition of the sale.
You want to move and take your home security equipment with you
You may want to bring your old smart devices with you to a new house when you move. It should be fairly easy to transfer, as most alarms and smart devices are wireless. You can remove each component by yourself or contact a professional installer to come and dismantle the system. Then an expert will set up the same components -- and any additions you need to make -- in your new place. The home security contract will stay the same.