Because not everyone will follow the security announcements of Debian very closely and as I love copy/pasting.
Debian has discovered a serious issue with openssl, that means your ssh key can be less secure than you expect.
Details at: http://wiki.debian.org/SSLkeys
What does it mean for GNOME? If you have a GNOME account, you need to check if you are hit by this bug, read the wiki page carefully and take appropriate action.
If you change your key, send the public part of it to accounts@gnome.org and do not forget to mention your username.