As for the contents of the update, SamMobile says it improves battery life and makes the watch more secure. There's also at least one report that says it brings the Galaxy Watch Active 2's One UI interface over to the device. We don't have a Gear S2 on hand to double-check this claim and there's no mention of a UI refresh in the changelog, so if you own a Gear S2, it's best not to get your hopes up too high before downloading the 6.79MB file.
That said, Samsung does have a history of supporting its older Tizen smartwatches. In May, for instance, it updated the Galaxy Watch, Gear S3 and Gear Sport to bring a bunch of Galaxy Watch Active features to those older devices. In any case, new interface or not, we won't complain about Samsung continuing to support one of its older products.