Some people want this feature for security or to have apps restart.
macOS has a lot of features that control when your Mac reduces the power it uses and how it behaves when it does. There’s a relatively hidden setting that automatically logs you out of the current macOS account after a chosen number of minutes idle. This doesn’t shut down your Mac, but it quits all open programs and makes the Mac inaccessible without a password unless you’ve turned on the Guest account.
The setting is found at > System Settings > Privacy & Security. Scroll down to the very bottom and click the Advanced button. This is where you’ll find “Log out automatically after inactivity.” If enabled, there’s also a field into which you can enter the amount of time desired. If it was a mystery as to why your Mac was logging out after inactivity—as it was to one Macworld reader—just disable that switch.
Read more at MacWorld.com