Typically, the reason to pair an iR device with some other Garmin device is the ability to remote control the iR device. This is easiest to understand with wearables, such as a fenix watch. Same principle applies to a cycling device like ZumoXT. You can leave the iR device in your bag or basket. You can control it (for example, start/stop tracking) and get notifications of incoming messages using the paired device.
Be aware of two things. The connection is short-range ANT+. The iR device still needs a decent sky view in order to function at all.
