If you're asking for the why, distributing a small (couple megabytes at most) patch is a lot easier than redistributing the entire file.
If you're asking for the how, see this handy guide:
http://www.srsfkn.biz/2008/11/11/mak...-with-xdelta3/
(xdelta3 is probably the best program for this; it works on both windows and unix, is easy to use and the executable size is small. You can of course use mkvmerge instead but it's a bit more annoying.)
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17:43:13 <~deculture> Also, TheFluff, you are so fucking slowpoke.jpg that people think we dropped the DVD's.
17:43:16 <~deculture> nice job, fag!
01:04:41 < Plorkyeran> it was annoying to typeset so it should be annoying to read