Seems your have either a 2011 or a 2012 Mac Mini, which would have originally come with OS X 10.7 or OS X 10.8 installed respectively, and it has been updated to 10.9.
No, you don't need to update to the latest operating system, now known as MacOS. Many, if not most, folks do if they have a compatible Mac Mini, but some prefer not to, for one reason or another.
If you do update, some of the improvements of MacOS 10.11 (but not all) will be available to you. It may, or may not, be of benefit to you, depending on what you do with the computer, and what apps (aka "software") you use.