Backports allow users to keep a stable base of an older version of Ubuntu
but be able to install newer versions of supported applications.

Say for example you are still a Hardy user which has one more year of
support. You got OpenOffice 2.x when you installed Hardy. Then it would
never allow you to upgrade to OOO 3.x. However if you enable the backports
then you can install 3.2 in Hardy 8.04. So you get the best of both worlds.
You get the stability that you need but can take advantage of improvements
to applications.

You can use the backports selectively allowing you to upgrade some
applications by browsing the list of applications online and downloading the
deb or generically to upgrade many packages by adding a line to your sources
in Synaptic.

See: https://help.ubuntu.com/community/UbuntuBackports

This is not any use to Lucid until Maverick comes out on 101010. But if you
are using Karmic or earlier then you can take advantage of backports.