the idea of turning of the light is meant especially for the beginner..the sun is a super strong omni light and to compensate it you need to balance the exposure with vray physical camera. THis balancing act creates a different effects with IES light even. and many users becomes even frustrated...
therefore so that you are not solving many things at the same time and you are only looking at IES light effects, its useful to turned off your sun and the camera. this time you only have to take care of your IES lighting. I have done lots of tutorial with VRAY IEs lighting and they are everywhere. One piece of advise,, units affects the ies data. vray working unit is inches. so be sure your unit is in inches... β