I see I am not the first to have thought of this. Here is a post from 3 years ago that has long since been closed:
forums.garmin.com/.../feature-requests-gear-tracking-duration-and-activities-search-gear-filter
Anyway, I have submitted it as an idea with some background as to why it would be useful:
Garmin Rally pedals
The rationale here is to that battery levels in Garmin Rally pedals can be monitored by adding the pedals as a piece of Gear by default to cycling activities. T
he current battery monitoring in the Rally pedals isn't ideal because only one battery is monitored and if the other pedal has a lower battery level, it will fail before the 10-20 hours. It would therefore be very helpful to know when to change them to avoid unplanned outages during rides.
Electronic Groupsets
Similarly with electronic groupsets it would be helpful if a reminder was provided as to when to charge the battery. By creating the battery as a piece of gear, and adding it by default to cycling activities, the Gear duration tracking can be used.
I'm sure there are other some uses in particular for battery power equipment that is expected to last some time.
HRM and other sensors