One of the administrative tools in Ubuntu is something called the Computer Janitor. It's a tool that is designed to take missing packages or other pieces of software that are no longer needed or in use and clean the system up. It's for newer users who might not be deleting or removing everything when taking software off the machine. While this sounds like a good software freeing up extra disk space, there are some concerns that one needs to know about it.
Generally speaking, Ubuntu software works pretty well since it's tested by real people. This software seems to have some bugs that you should be aware of.
One huge problem is that Computer Janitor tends to remove packages that you have installed to the system that are not found in the repositories. For the most part, many open source applications will be found on the repositories either put there by Debian or by Canonical which programs Ubuntu. Sometimes you need other packages that are not part of this software collection. If it's from another source, Computer Janitor wants to remove it from the machine. Many run this application and find their software is gone.
At other times this tool will remove very small files that are not used too often. While it seems that it is doing a good job in saving you some space, there are some that have run into issues where necessary files have been removed.
For the time being, consider skipping using this tool. In the coming updates and upgrades, a tool like this can be improved, but at the time being ignore using it on your Ubuntu system.