A solution to the failed Catalina installation attempt with the message: "Some APFS cryptographic state information was unexpectedly unavailable. : (-69461)"...and another question

After trying unsuccessfully to install Catalina and repeatedly getting the error message above, I spent a long time with Apple Support who helped me to reinstall Mojave (which I had deleted!) and then instruct me in installing Catalina. This was a clean install and the same thing happened. After Apple Support closed, I did a web search and finally found a solution on Reddit that worked! It involved a little Terminal use but if I could do it, anyone can. I have attached a screenshot of the procedure. My question follows the screenshot...

The question: after installing Catalina, there is an item under Locations in my Finder sidebar with an icon that looks like a hard disk named VM. When I click in it, there is a file named "sleepimage." What is it, how did it get there, is it important, and should I delete, leave alone or do something else?

I hope the solution above helps somebody...and I hope somebody can answer my questions!

Thanks.