There might come a time when you want to know if you have already installed a certain package on your RHEL 8. An application to be manually installed might require certain dependencies to work so you'll have to check beforehand if these are satisfied. Or you might want to compile something and want to make sure you have all the right header libraries on your system.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-list-installed-packages-on-redhat-8