The default Ubuntu installation process places all of the files in a single disk partition. However, it may be desirable to use multiple partitions. In particular you might want to have
/home in its own partition and have yet another partition for the swap area. This way you could install another Linux distribution and have it share the swap area and
/home, so you can save swap space and share your data files between the distributions. I have done this successfully on another system. In that case I created the multiple partitions when installing the distributions. For the Ubuntu installer it is the “something else” option when you choose how you are going to use your disk. It is fussier than following the defaults, but easier than the Slackware disk partitioning I used to do.
What if you followed the default Ubuntu install process and want to create additional partitions after the fact? That is what I wanted to do before I ended up Rebuilding a Linux System.