Printing is really a kind of conversion process. That’s a bit of a clue as to its current usefulness. You’ll note that’s a Windows 7 style dialog “Print to file” doesn’t appear on any of my Windows 10 installations. This option may appear in different locations on different print dialogs, or it may not appear at all. Here’s an example of the option we’re talking about: Print to file option in a print dialog.
It’s highly unlikely you’ll ever have a specific need.
If you see it, which is rare these days, then unless you know you have a specific need for it, just ignore it. The super-short version is that it’s somewhat of a relic.