I use the Guest session in 16.04 (I only use LTS releases). I use it so that I can safely hand out my laptop to visitors without worrying that they’ll break my machine or leave the SSD full of crud. Sometimes I also want my children to use my desktop workstation without worrying that they’ll mess something up. I also have a couple of old spare laptops that I can safely lend to friends on a longer-term basis; again they use the Guest account (“Andrew, my Macbook broke, have you got a spare laptop I can borrow for a fortnight?” “Sure…”)
Typically the guests will browse to a website and, occasionally, print something out, usually a web page or a PDF. Very occasionally they will want to open an MS-Word document in LibreOffice and perhaps print that. Also very occasionally they will want to access a document, MP3 or video on a USB stick, and perhaps copy that data to another removable storage device.
I find it annoying that it is difficult to customise the Guest account, for instance to remove some icons from the sidebar so that they don’t confuse newbie visitors, or to start the web browser automatically. I did once write a script so that root could zip up a Visitor (not guest) user home directory, which would then be cleared down and unzipped by /etc/gdm/PostLogin/Default each time Visitor logged in. I then set an empty password (no password) for the Visitor user and disabled the Guest login, so that GDM showed only my login and Visitor as options. This allowed me to customise Visitor’s desktop, safe in the knowledge that it would be reset to my preferences at each login, but without changing the defaults so it didn’t affect any new normal users I might create.