As with most technologies Sitecore versions are only supported for a few years after their launch. During this time (Usually between 3 and 4 years), you will benefit from a range of updates, bug fixes and security patches. However, sit on the same version for too long without updating you risk leaving your site venerable to security flaws.
The other risk is that older versions will fall behind any advances in technology. For example, Sitecore Cortex was released for Sitecore 9. Those on Sitecore 8.2 or below won’t be able to make use of this.
Learn more about upgrading and see if now is the time to move to Sitecore 9.1.
