I don't often use multiple monitors at once, but when I do, I'm using multiple computers too. I'm mainly doing this to test websites on multiple operating systems as my desktop machine runs Linux Mint and my work laptop has Windows 10 on it. Of course I could use Virtualbox to run any number of operating systems, but having a separate device makes things a bit more simple for me. What I despise is using the laptop keyboard and the trackpad. Wouldn't it be nice if I could use the desktop keyboard and mouse on the laptop too?
I was really excited that I would be able to use a Mac for the first time in my life at the new job I landed. After using Windows and Linux for more than 20 years in parallel this seemed to be a great opportunity to try something new.