Suppose I have a backup application that performs scheduled backups of my Exchange Server using the VSS API: Why does the backup application need to backup all the log files as well as the .edb file if the Exchange VSS writer already causes all the log file transactions to be committed to the .edb file prior to backup. It seems to be double data that is being backed up?
SnapOverflow
This is merely a guess, however it could be so you have absolutely everything in your backup, including any transactions after the full backup started.
If for example your backup takes 6 hours to run, with a transaction log backup at the end of the full backup, you have also just backed up an extra 6 hours worth of data for very little extra required time.