China views the US as an adversary. They would very much like to reduce America's sphere of influence and are cognizant of the fact that we might end up in a war in the medium term future. The US bleeding immense amounts of money and military assets in Iran is great for China's relative strength; it's in their interest to increase those costs in ways that don't escalate immediate tensions with America. Sharing targeting and other intel is one of the more effective ways of doing that.

China is allied with Iran and has long term trade relations with the country, importing large amounts of oil from Iran, and supplying Iran with air defense systems and other military and economic support.

China also views the US as a strategic rival and would love the opportunity to take us down a peg.

Don't think that America's strategic opponents -- Russia, North Korea, Iran, China, Algeria do not provide some mutual support, even if for purposes of survival, and view the US as threat. We have already taken out Venezuela, Lybia, Syria and flipped Armenia. Cuba, Iran are next on our radar, but we are active all over the world trying to flip pro-Russian/pro-Chinese governments to pro-US governments.

China depends on Iranian oil, I'm pretty sure they view that war as an attack on China by proxy, it's a big deal.

Because how much oil they (used to) get from Iran