When do I need *-device-width?

[shiprepairwoman]

I'm creating responsive web app for desktop and mobile devices. My problem is I don't know when I need to use \[code\]*-device-width\[/code\]. Pls explain usecases for \[code\]*-device-width\[/code\]. Why should I use it instead of \[code\]*-width\[/code\]?