If you are connected to internet during Ubuntu (or any of its variants) installer, you have the option "Download updates while installing".
But even if you check this option, there are updates pending when you boot on your fresh installed system. So my question is, what is the reasoning to decide which packages can be updated during installation and which packages can not?
The option only DOWNLOADS the updates. It does not install them. I never check the option because I don't find that it always works. Also, why not just do it after the install because then you can start customizing the system during the updates.
And the other option to install multimedia codecs is VERY helpful because then you can do most multimedia stuff without having to install other codecs.
The "Download updates while installing" tickbox is somewhat of a misnomer. As you can see from the description below it, this tickbox only downloads a few packages in order to enable flash and mp3 etc to function.
That option asks if you want to install closed source third-party software for MP3 playback and Flash. I would strongly suggest that you check this option.
Also, I think that it downloads files that are needed for the installer, in-case there are updates available since the ISO was made, to make the installation go smooth.