Many have mentioned HDR which can be worth it but also Dolby vision too.

Another benefit in the UHD releases is often times you'll get better or remastered Dolby Atmos audio or DTS-X for object based audio. If you have a setup that can leverage this it is often worth it alone.

Another complexity is not all UHD disc transfers are true 4K source material and may often be upscaled 2K source. There are websites that track this in case it matters.