The description you link to is pretty clear:
True if the process is running in design mode; otherwise false.
Remarks
Use the DesignModeEnabled property when your custom types require special logic when running in a visual designer.
I.e. it allows your code to determine if it is running in the Visual Studio designer or on a live form.
If it's on a designer, you might want to fill it with dummy data, or put a border round it, or anything else to make the designers job easier, but that won't affect the "Live" form.