The same bug can crop up in different places.
Imagine a bug in a video driver. Not only do the video driver maintainers need to fix the bug in their code but so do the people who package that driver for Ubuntu and other distros.
Of course, the version and packaging of that driver may differ between different releases of a particularly distro.
And the bug may initially be reported against X.org or some other part of the operating system.
Launchpad helps manage this by allowing bug reports to relate to more than one context.
So, if you suspect that an existing bug also affects another project or distribution you can add it to the report. That bug report, with the same number and comment history, will then show up just like any other bug report for that project/distro.
Launchpad can track bugs for: