Locking user for LO Cookpit to transport changes with new field?

We have loaded much LO Cookpit data to BW DSOs already. Now we added some new fields to this LO Cookpit application and we often hear that Before R/3 transports, we should Lock the users (transaction lock) and drain the delta queue into BW till we get 0 LUWs in RSA7 for that datasource in R/3 system.

Could some experts here tell us why it's necessary to lock user input and empty the extraction queue/delta queue in R3?

Really appreciate anyone's input!