Why do you see 'removing additional packages' during the Ubuntu installation?

TLDR; It's basically just a apt-get autoremove performed after installation.

Here's what happens on installation time using a desktop installer (this answer is not valid for server or alternate installers):

1. A sort of preinstalled Ubuntu image (/casper/filesystem.squashfs on the CD) is extracted onto your hard drive. It's like taking a shortcut, because it's all in a predefined state and does not require all packages to be installed one by one (opposed to the debootstrap process in the server/alternate installers).
2. After extraction of the image, the installer chroots into the target and will apply all changes you entered during installation: user account, language, timezone, etc. It will then also apply updates.
3. The state of the target filesystem (almost finished installation) after all this will probably result in some packages not needed for your system. E.g. older kernel that is not applicable anymore due to updates, or you didn't choose to use LVM so all packages related to that can be removed. (this is just hypothetical - I did not check if this actually performs the removal of LVM.) It's not much different from applying a large stack of package updates that result in some packages removed using apt-get autoremove.

Though there can be several reason, I think those packages were removed as they are unneeded now. They are installed because they were on the installation media. Now new updates came in and mark those item as not required. So ubuntu is removing it.

Another reason could be, for installation purpose ubuntu installed some packages which are not used for normal day to day usage. So it is cleaning them up.

There are packages present on the Live CD image which may get uninstalled after the image is copied to your hard drive. One example I can think of is gparted, it is present on the Live CD session but not available on a newly installed system. Other packages that this occurs with are usually filesystem related, for example LVM is present on the Live CD in case you want to use LVM partitioning on your installation. If you don't use this then there is no reason to have the LVM package on your installed system so it is removed. I think that the unused language packs get removed as well.