Before OS X 10.6 Snow Leopard, I used to tell my clients not to run Software Update. I kept up on whether any of the updates were problematic and dealt with all updates whenever I saw my clients. Then in Snow Leopard, Apple quietly started offering virus and malware protection. Very quietly. With Snow Leopard, Apple started to include a built-in system that detects malicious software and attempts to protect users from inadvertently damaging their computers.
Apple began checking files against known malware, pulling from a list of malware definitions that they kept up-to-date through Software Update. That changed everything, and I began to strongly recommend that Software Update be run daily.
Fast forward to February, 2014, when Apple discontinued support for Snow Leopard. Most folks think that there won't be any future software updates for 10.6, and that means the malware definitions list won't be updated either.
OS X 10.7 Lion continues to be supported and get those malware definitions updated.
OS X 10.8 Mountain Lion was the first version of OS X to "phone home" to Apple daily and automatically check for any required security updates. Some of these updates install automatically, while other require a restart.
Obviously, many users ignore Software Update's notifications, thereby allowing OS X to become out of date. In doing so, however, users run the risk of leaving themselves vulnerable to well-known exploits that have long since been patched. That Apple has reduced a source of friction by installing security patches is a major plus in my view.
There are larger ramifications, though. Since Snow Leopard won't get malware updates, to be protected means running virus protection on your Mac. Although there is plenty of free and paid software, there are often performance and production issues, and sometimes software conflicts.
Moving up the food chain to Lion allows you to manually run Software Update (which I recommend setting to be checked daily). Mountain Lion and Mavericks get updated by Apple, but you should also set Software Update to check for updates daily.