You can't use the Finder to copy an entire Mac OS X boot drive to another partition - well, you can, but the result won't be a bootable system. CCC can perform such a copy, preserving not only all data but also permissions, ownership, and bootability (hence the name, "Cloner"). Thus, you could use CCC to make a perfect, usable copy of your system to another drive before installing some possibly dangerous system update, or before travelling with your computer or handing it in for repairs.
CCC can also perform backup of selected items, and it synchronizes between the original and the backed up copy, meaning that in future backups it will copy only items that have changed since the previous backup.
New features in CCC3 include: you can make block-level copies of one partition to another; you can back up across a network; and scheduling and specification of backups is improved.
CCC is free. Mike Bombich asks only that you consider donating something, directly or via PayPal, if CCC proves to be of value to you. Backups are important. With CCC3, you've less excuse for not having one than ever before. Don't say we didn't warn you.