Currently, OT arm/chest/wrist monitors (which you pay for separately) take your heart rate, send it to their servers, process it and display your stats on the in-studio TV and to the ios app.
With the new upcoming dongle, it looks like they are moving away from their proprietary monitors to Apple Watch for capturing data (treadmill, rowing and TRX strap exercises) but they still want the dongle to send the Apple Watch data to their servers for processing.
Now, they could have done the following:
1. Integrate Apple Watch data to their ios app and do the processing on their servers. (No dongle required. Apple Watch captures all the data OT needs to display the stats in their format).
2. If at all they cannot consume Apple Watch data in the given format for some reason, they can install dongles to their equipment which can convert Apple Watch data into a format they need and process it.
3. Instead, they decided to put the dongle burden on the user/consumer (just like OT forced their users to buy their OT-specific monitors). That way, the consumer is paying for it instead of OT paying for their equipment upgrade/maintenance.