I sometimes have to use more than one Garmin device to get a complete set of data during a workout. My phone doesn't have ANT+, so I can't use a Heart Rate monitor with Garmin FIT. But my FR70 doesn't track speed, position, elevation, etc.
Both can upload to Garmin Connect, so I'd like to select both activities and merge them into one (with a choice for which data set "wins" if the same data type exists in both activities).
Seems like you could do this across the time dimension as well -- sometimes I capture multiple legs of a long cycling day as individual activities on my device. I'd like to consolidate these into a single activity after they've been uploaded to Garmin Connect.