They are used to fill gaps in coverage where you can't add another subsite to an existing simulcast site (this is typically done for licensing requirements, or to segment specific traffic onto a new site). For example, I am in no way in the know, but if I had to guess State of Delaware had to put an IR there on seperate frequencies to fill a coverage gap, but couldn't add another subsite to the New Castle simulcast because the frequencies would have interferred with something in PA, MD, or NJ. So, new site with new frequencies to enhance coverage.
Within the radios themselves, you can program which sites to prefer, if I had to take a wild guess based on the location of that IR, subscribers won't affiliate until they have really crummy coverage from the existing New Castle simulcast.
Based on my experiences (and this is an opinion + strongly educated guess) I would do what you can to pick up the New Castle Simulcast, including mapping the closes tower location for that simulcast and point a Yagi at is as I am much more likely to pick up traffic on that than the IR, which is likely just picking up folks who are in the fringe area.