One reason to look at upgrades is the expectation of useful new features.
As a for example, something that I'd value in the NVR web GUI is a playback timeline for continuous recording that is marked with the camera events such as intrusion detection and line crossing detection, such as exists in QNAP Surveillance Station. It does exist in the NVR VGA/HDMI access, so it might be reasonable to expect the code to be added to the web GUI as a useful enhancement. That expectation has not yet been met.
We all know that we can't rely much on Hikvision's change logs to document all useful changes, they don't go into enough detail for people to make an informed decision about whether an upgrade would be worth doing.
Significant changes that I've seen in recent NVR firmware versions include for example dropping ONVIF support, enhanced support for configuring actions for smart video analytics events, HTTPS support, channel zero support, digital zoom on playback, enforced improved authentication measures, a fresh-looking style etc.