What difference between packages

Some reasons for doing this:

* Ubuntu does releases at different time from Debian (much more often).
This means that they sync with Debian at certain point and then kind
of "branch" that together-working set of package versions when stabilizing
it for a release (with security etc fixes). When work for the next Ubuntu
release starts, they again sync with Debian

* Binary packages re-compilation (if that's what you meant
with "re-packaging") is a must, just to make that everything really is
buildable and works together...

* There are also some differences in Ubuntu base system (Upstart several
releases before Debian, Python as an essential package which IMHO was
mistake, essentials should be minimized, not frivolously expanded etc).