Are you asking why people use non-physical switches in general? Or why they specifically use the “virtual” DTH? The specific answer is just because it runs locally.
The general answer is that there are a number of different reasons.
One of the most common is that there is something (which could be a third-party system like Amazon Alexa or a third-party security system or a third-party automated sprinkler system) Which can recognize that a switch can come on in smartthings. Maybe through IFTTT, maybe through a direct integration. So you create a non-physical device, control it using SmartThings logic, and that switch going on or off is picked up by the other system so it triggers events over there. Or the other way around, the other system turns that switch on or off and then you have SmartThings do some set of events. Again, very commonly used with both smart speakers and with IFTTT.
Another use for non-physical devices is to set up a timer on a particular switch. You set the conditions for the non physical Device so that you can still use the actual physical device in other ways at other times.
http://thingsthataresmart.wiki/index.php?title=How_to_create_a_virtual_timer_for_a_light
Still another use before we had webcore was to have one non-physical device stand in for a whole group of actual devices, such as with the motion sensor zone manager smartapp. Nowadays I think more people use webcore and just list out the whole group of devices, but sometimes the non-physical representation of the group is still useful, particularly when working with smart home monitor.
Did that help any? Or did I misunderstand the question? 