I assume you're referring to the infrared searing burner on many of the newer gas grills. It is, as you can guess from the name, there to provide a rapid sear (most gas grills (especially at the low end) just can't produce the necessary BTUs to get a quick sear in a piece of meat) accelerating the surface maillard reaction as well as sealing in juices. The infrareds can get to 900 degrees or more (my pricey Weber gas grill can get to 700 degrees, but that's about it) very quickly and get a very rapid sear, so you retain more juice in the meat. On the downside, it eats into cooing surface area (and doesn't really function so well for just grilling)
i can't speak to the usefulness of infrared burners from all manufacturers, but the one I have from Lynx is quite useful. It gets very hot, much hotter than the other burners or my Weber. I can't keep my hand above the grates for even 2 seconds when it's going. I use it for practically everything, since most of my grilling is high-heat cooking. It's also adjustable, so you can lower the temp for chicken or other items that don't require the high heat. I don't think every manufacturer makes adjustable infrared burners, so that might be a consideration.
I haven't found a downside, and would recommend it.