When you are working with strobes, one of the downsides is not being able to visualize how your light is going to fall on your subject. Almost all modern strobes have modeling lights built in that will allow you the opportunity to see what you are going to get lighting wise before you take a photo. Utilizing the modeling lights give you the benefit of being able to make big changes to your lighting setup without having to take test shots constantly. This is a huge benefit when working with a client because you don’t have to appear as if you are second guessing yourself while they sit waiting for you to get it just right which further increases their confidence in you and your abilities.
Beyond being able to use modeling lights to pre-visualize, they also have some other handy uses. If you are out shooting on location and the sun is going down, but you are trying to squeeze the last few shots off to get that colorful sky in the background, having the modeling light on makes focusing on your subject much easier. The same concept applies if you are shooting at night or in a really dark location. Another option is using them as a continuous light source and not using the strobe function. This is great when you want to shoot with a shallower depth of field and your strobe is too powerful. There are also times when shooting on location and you might not have room to use a soft box or the modifiers that you want to or have with you.
The one major thing to be mindful of if you are shooting on location and using your modeling light is that it will drain your battery much quicker than if you have it turned off.
As always, the best thing to do is to take your light out and practice, practice, practice.