Die Frage wird ja im Moment oft diskutiert. Laut https://politics.stackexchange.com/questions/71027/what-is-strategic-about-chernobyl liegt es einfach auf der kürzesten Verbindung von Belarus nach Kiew. Hinzu kommt
Controlling the Chernobyl Nuclear Power Plant is one of the better thought-out parts of that. The idea will be to say that Russia is in control of this dangerous place, which would be a threat to much of Europe if it were damaged. This allows Russia to be portrayed as selflessly assuming responsibility, with the (false) subtext that the Ukrainian government was not being responsible.
It was also an easy place to take and to hold, because the Ukrainians would not be fool enough to fight for it, given the potential consequences of using heavy weapons there. It also means that should the Russians start to do badly in the conflict, they have a threat to employ against Ukraine, which would have to live with the consequences of any breach of the containment.