I just had this idea right now and had to write it quickly. 

Now this isn't necessarily part of the installer, but the first start install straight after the installer. Ever since I saw the Mac OS X Snow Leopard intro, I've loved it and I always thought that it was a shame that Apple never continued that. Perhaps they thought they couldn't top it. Maybe it's like their Calculator App on iPad issue.

Anyway I've never used MacOS X but I've really enjoyed the intro for all the years I've known it, and have wanted to replicate it in some way. Now that I'm starting all my software projects, I've got a non-negligible reason to do it. The game Yellow is another, more recent example I can think of.

The intro also bridges the gap between install experiences on desktop and mobile, as custom installers aren't a thing on the latter.