Many computers these days come with two hard drives, one SSD for fast boot speeds, and one that can be used for storage. My Dell G5 gaming laptop is a great example with a 128GB NAND SSD and a 1TB SSD ...
External hard drives can be moved between systems, reformatted at will, and repurposed without risking the data or stability ...
I currently have five external drives attached to my System76 Thelio, each of which serves a different purpose. I have one to house virtual machines, another for music files, another for backups, one ...