Disc 1:
-vanilla Skyrim updated to version 1.9
Disc 2:
-Brings you to a menu to install each of the 3 DLCs
-This is also a good way to share the DLC with friends who don't have it, as all you have to do is install the DLC from the disc on their Xbox
Typically games do this when they release add on packs, ultimate editions, or goty, then those get ripped and uploaded so people can download them.
Borderlnds
1 &
2 are examples of addon packs
Dragon Age Origins is an example of Ultimate Editions
Fallout 3/NV, Skyrim, and Oblivion are good examples of GOTY games that did this (
here's an unboxing of Skyrim Legendary so you can see how it works)
There has also been instances of arcade games going the same route, like the
Worms series
These files typically either get ripped or just copied from the hard drive once they are installed over to a USB, then they are uploaded to a site, and then downloaded by many people.
(Also, all of these links are either to Amazon or YouTube)