Sierra on a mid-2011 iMac

Having the latest OS X installed so you can run the latest versions of apps such as Safari.

macOS - How to Upgrade - Apple

Before you upgrade make sure to backup all your important data.

And, check your third party software, ie., printer drivers to make sure they are compatible with macOS Sierra 10.12.

Another consideration, seems that the latest trend, though not a historically hard and fast rule, for Security Updates from Apple is the current OS, plus the two most recent versions behind that, older than that are considered retired and no longer anticipated to be updated. (Sierra, El Capitan, Yosemite, would be the currently supported releases by that rule-of-thumb)