At the present time, an Alexa routine (not a smartthings routine, but rather one created in Amazon‘s Alexa app) can only be triggered from a contact sensor, motion sensor, lock, flic button, or echo brand button. It cannot be triggered by a switch of any kind.
https://www.amazon.com/gp/help/customer/display.html?ref_=hp_bc_nav&nodeId=GCCRGDPHL7L9W7TJ
So many people use a virtual device which is both a contact sensor and a switch. Alexa routines can be triggered when the contact sensor opens or closes. And you can use the switch from a smartthings routine or other smartthings automation, or by voice. So the virtual device has both the capability switch and the capability contact sensor. When you turn on the switch, the sensor opens. When you turn off the switch, the sensor closes. This turned out to be a great way to integrate smartthings automations and Alexa routines.
For this reason, most of the community created Edge Driver virtual devices have both a switch and a sensor capability. 
See the community FAQ for more details:
FAQ: Can I trigger an Echo Action without Speaking to It?
Use Cases
As far as use cases: using this method, you can take any command that you would speak to an echo device and instead automate it from smartthings. For example, suppose you have a sprinkler system which doesn’t have any integration method with smartthings but which does work with Alexa.
If you create a virtual device which is both a contact sensor and a switch, you can have whatever logic you want on the smartthings side (Check temperature, weather, time of day, whether you are home, whether the pet door is closed, etc.) and use that to create an automation which turns on the switch part of your virtual device in order to trigger an Alexa routine to start the sprinkler. 
So Alexa becomes an intermediary between SmartThings and a third-party device which does integrate with Alexa but doesn’t normally have an integration with smartthings. Very useful.
It’s also a good way to create a simple smartthings integration for echo devices, including fire TV devices. And it’s also a popular way to have an echo device make an announcement or play a chime when a door is opened or a particular person arrives home.
Basically almost anything you could say to an echo can instead be added to a smartthings integration via a virtual device of this type.