A. You are backing up to tape and your tape device performs its own compression.
B. You are making image copies, and you have not enabled tablespace compression.
C. You are using disk-based backups and disk space in your Flash Recovery Area, or other disk-based backup destinations are limited.
D. You are performing your backups to some device over a network where reduced network bandwidth is more important than CPU usage.
E. You are using some archival backup media, such as CD or DVD, where reducing backup sizes saves  media costs and archival storage.