The exact reasons aren't entirely clear, originally they hated NS because it allowed Europe to ignore Ukraine in the gas trade which left them more exposed. By the time of the full scale war I would bet the reason was more "fuck Russia" than anything more carefully reasoned.
https://www.reuters.com/business/energy/russia-china-bless-v...
So Russia can now export gas, get foreign currency, and buy weapons with the money. I do not see any strategic wins here.
Additionally, China gets an economic boost. That is a sublime strategy.
The strategic win in bombing Nord Stream was that Ukraine finally got Europe on their side. Before NS was blown up many countries, especially Germany were sitting on the fence, reluctant to give Ukraine any help. They were hoping for Ukraine to lose the war quickly, then they would give Putin some slap on the wrist punishment, and return to "business as usual" with Russia. Nord Stream being destroyed removed the biggest incentive for doing that.