|Imagine that you have a slow connection, and a fast CPU.
Now, imagine you want to download that brand new X release — call it version 4.1.0-1.
If you grab all of the X 4.1.0-1 binary packages, it’s probably about 30 MB. (I haven’t actually done the math on that.)
If you grab the X debianized source, it’s about 50 MB. 48 MB or so of that is the upstream tarball; the other 2 MB or so is the Debian changes.
Building X from source will probably take a few hours of CPU time. Downloading 30 MB or 50 MB on our hypothetical connection could take a couple days.
Now, Branden (being the hard-working fiend that he is) puts out X 4.1.0-2 packages. And you want to upgrade.
If you download the binary packages, it’s another 30 MB.
If you download the source, though, it’s only 2 MB. You already have the xfree86 4.1.0 upstream tarball, so you only have to get the new .diff.gz and .dsc files.