I'm ON Catalina right now, and I didn't even know that. (I typically use my Linux laptop for everything... dunno why I decided to use my Macbook Pro today) I had to triple-swipe right just to see if was really gone. Yup. It's gone. Dang.
So, worth noting, I'm sure I saw plenty of these explanations over the years, and it still took watching Steve Jobs's demo before I "got" it.
But: it's a workspace for quick glances that don't take you out of the flow of what you're working on. If you've ever kept a paper notepad, or a calculator, or a small tablet on your desk for reference while using your computer, Dashboard is that, but even better because it can be activated and deactivated more quickly than physically looking down at your desk.
(Make sure the Dashboard is set "As Overlay" if you're ever on a system that has it. Modern-ish macOS defaults to "As Space", which reeks of a setting created by people who didn't actually use the Dashboard.)
There were several widgets I used a lot - a fab BBC radio widget rage let me listen to radio channels (bring up the UI instantly and then dismiss) same for a widget that controlled my MAMP servers and a web clipping of a page showing the status of various systems I monitored
A couple weeks ago, I watched the keynote of Steve Jobs introducing Tiger, and when he demo'd the Dashboard, I had this moment of dawning realization:
"Oh, so that's what you're supposed to use that for. That is... actually completely brilliant. Yes. I want that. Now."
I'm now using the Dashboard quite heavily. It's great.
It got even better a few days later when I watched the Leopard keynote and discovered how you're supposed to use Web Clips.