When you have added new persistent storage to a system
When you have added new persistent storage definitions to /etc/fstab on your system. A good practice is to see if those definitions work, before you reboot your system.
Doing so prevents getting stuck in a situation where your system does not boot up, due to faulty /etc/fstab definitions.
Practice
After having added a new definition to /etc/fstab test it by running:
If your definitions are without issues, the command with exit without any output, as such:
If there are issues which the system will encounter doing boot, you will find those issues before rebooting it, allowing you to fix the issues before the system is rebooted.