Without this you would e.g.:

1. Set up MoveTables create
2. Set up LookupVindex create for 1 or more lookup vindexes
3. RunMoveTables switchtraffic
4. Run LookupVindex externalize for the lookup vindexes
5. Oh no, something went wrong, have to run MoveTables reversetraffic
6. Because you already did externalize, you need to rebuild the lookup vindexes from scratch or manually reconstruct those workflows

Ideally as part of step 5 you could also run LookupVindex internalize which would be the rough equivalent of reversetraffic for LookupVindex workflows (on externalize the ownership/management of the lookup vindex transfers from vreplication to vtgate, and internalize would be the opposite to get us back to where we started).