Each major release has a list of upgrades in its release notes - you can go over those and see if any of those changes are important to you. There's no way to guess which may or may not match your specific implementation and needs/wants. For QlikView, this is once per year, so you only have to look at two versions at the moment (and I would not recommend installing 12.3 November 2018 until some initial bugs are resolved in service releases). For Sense, the 10-week release schedule means there's more to cover, and Qlik seems to put a lot more effort into adding functionality to Sense than they do to View, so there's more incentive to upgrade regularly.
Insofar as upgrading service release builds - those are typically just bug-solving, so if you aren't experiencing bugs, you don't really have to upgrade.
My own personal recommendation: Upgrade Sense as often as you're willing to test new versions, and upgrade QlikView only if the new version has something you want (features or resolution to specific bugs) or your old version is out of support (12.1 has another year of support left, so plenty of time if you want to wait for 12.3 to stabilize).
