Be able to lend a bike or to manually add mileage to a bike
You exchange bikes with a friend for a ride or you lend one of your bikes to a friend.
Those kms are not recorded and break all your components mileages.
1) Be able to note that you ride with a bike from one of your Strava friend. (this of course requires your friend to install and use Strava, which is a good thing :-) ).
2) Manually add mileage to a bike at a given date (which would then affect the components on that date).