In a perfect world, your systems perform a great backup every night, and you've sized your transaction log drive space adequately so you never run low on space. In the real world, however, things are rarely perfect.
Tape backups fail, and server requirements can change faster than your hardware budget allows. In the real world, you might find that you need to move your transaction logs—and fast.
You could free up disk space temporarily by copying some of the older logs to a separate drive. This could work, at least for a while, and you don't have to shut down the information store to do it.
However, the problem with this approach becomes apparent if you need to do a restore. The log files are no longer where the system expects them to be, and therefore the system can't perform the restore.
You can hopefully move all of the transaction logs to a drive with enough space to complete the restore. Otherwise, you're stuck with a point-in-time restore, and your transaction logs are useless anyway.
Whenever possible, we recommend moving your transaction logs using the tools provided by Exchange. It's the safest, surest way to move the logs.
You can move Exchange 5.5 transaction logs (and databases) using the Performance Wizard; Exchange 2000 and Exchange 2003 use the Exchange System Manager to change the paths of files. While using these tools may take a little longer and cause a brief amount of downtime up front, the alternative—possible data loss—is far less desirable.