The US and Soviets were involved in tons of nuclear tests in the 50's and 60's. A ton of their tests were very straightforward. If they used a nuclear device against an atoll it was to test the effects of a bomb on an atoll. Underwater tests probably were done to see how submarines reacted to nuclear torpedos. The Hawaii tests were able to measure antenna effect. Every device test was probably intended to measure an entire class of new information.

The Soviets were no different. They launched satellites into orbit well ahead of any new tests. But the tests. They knew their nukes worked. They had the option to test them in the Pacific or the deserts of Kazakhstan but they dropped devices in the taiga. These were 'atmospheric nuclear tests' and Im pretty sure now that is somewhat of a euphemism. They were not testing the nukes they were testing the atmosphere.

You look at whats commonly known. They had satellites able to measure the long term effects of detonation. They already knew their devices worked. They had openly expressed interest in nuclear mega-engineering. They had a view that the taiga was worthless if covered in permafrost. They wanted to see if the weather changes caused by a nuclear bomb would be permenant, and if so, exploitable.