If they control the browser they can potentially intercept everything you see and do. Banking info, whether you looked up Tianmen square, who you talk to, who you trust... They can also infer personality from your browsing history. Looked at clown outfits just before watching porn? Maybe you'll get a letter blackmailing you or else they'll divulge your fetish to your family and loved ones. This is not fiction - China has been caught doing this to political personalities, and those are only the ones that failed.

The browser can also serve as a gateway for them to install persistent monitoring software on your OS, or turning your machine into part of a botnet.

They can correlate that information with your other browsing habits and start to form a picture about the sort of person who shares your interests, regardless of how bizarre those might be.

It's not an exact science because everyone is different, but once they have that picture they can start pushing the buttons of one person like and derive some conclusions about how the rest of that cohort, including you, will react.

They don't even have to be successful all of the time. Just more than would be expected from random chance.

A stone in the right place can divert the mightiest of rivers.

I think concerns about China in specific are overblown.
That being said, what we've learned about the topic from US tracking programs (slight chuckle at China having scope or abilities beyond anyone else in that regard) is that all information can be fed into what is essentially a statistical model of interests, behaviors, expressed opinions, and contacts.
From that, you can determine a few things that are specifically "useful".

The first useful thing is the ability to tell if someone's behavior has changed in an unexpected way. If someone starts talking to someone new via text message and they "shouldn't" know each other (no common acquaintances, never at the same place at the same time, no shared interests) you have an anomaly that can be processed further.

The next useful thing is once you have this model of expected behavior you can start modeling stuff like "A talked to B, B to C and then C changed behavior. A talked to D and D talked to E, and E changed behavior", and more or less direct chains.
This effectively tells you that A is influencing the behaviors of C and D. By tracking how influence (and money and stuff) flows through a network of people, you can extrapolate things like leadership, communication pathways, and material support pipelines. If you're the US, you can then send a seal team to shoot someone.

If you're, supposedly, anyone doing this you can more selectively target people for influence based on the reach that it'll have, use your models to target them better, and generally improve the quality of your attempted influence.

I personally have my doubts it's being used that way because it's just as effective and far cheaper to hire a public opinion research group to pay a significant sample of people $5 to figure out how to make better propaganda, and then like 75¢ each to get Facebook to target the right people.
It's really only valuable if you eventually care about an individual. Most unfortunate privacy violations are aggregates.

Even if it's not directly actionable or a threat, you should still be wary about letting your browsing habits leak because the information can much more plausibly be used for phishing purposes.
If you just bought some clown outfits and get an email about your clown plants being held at customs you're a lot more likely to click to figure out what's going on.