The main purpose is simply to contain data into a limited space in the drive.
For example, if your ever growing music collection fills up the entire drive, you may find one day that your computer won't boot properly or is unable to install new updates. By creating a partition for user data, only your partition would fill up, without affecting the rest of the system.
Another example may come from misbehaving processes that end up writing a ton of logs uncontrollably, which over time can lead to a similar result, or Yet another example when installing too many applications using Flatpak packages. This is a special package format that has one particular downside in that it takes up a lot of space in the drive.
There's is also another advantage of having a dedicated partition for your personal files: if you need to re-install Zorin OS or another Linux distribution, you can do so without having to erase your personal files. This makes it much easier when "distro hopping" or re-installing just for the sake of having a fresh start.