Imo it's pretty transparent, it's indeed all about ads. 70-80% of Google's revenue is from ads, that says it all imo.
First of all because of search: If you type something in the search bar of your browser, and that takes you to Google, you see all those ads and Google makes a lot of money.
Second of all because the browser is the entry point to the web. If you browse the web, the chance that you come across Google Adsense ads is very high, in other words, if you browse the web, Google makes money.
Browsers can control what you see, they can have ad blockers, they can replace ads (like the shady business Brave tried at some point), but also change the extension API so ad blockers are less effective (see manifest v3).
Conclusion: Controlling how people browse the internet is highly valuable as direct money maker (search ads) but also to make sure nobody but you can mess with 70-80% of your revenue. That alone is worth every dollar they spend on it.
Microsoft has Bing (but also based on chromium so less investment). Apple needs a browser for their devices, and gets 20B from Google to make it the default search engine (again, if Google can serve more ads, it makes more money). I don't know if Safari is well funded, they lag behind a bit currently.
Edit: Apple also has motivations btw. They have been lagging on implementing a lot of the features in Safari IOS that would make webapps more capable of replacing native apps, the App Store that Apple makes tons of money on... If you allow other browser you don't control that, so you need your own.
Second part why Google might want to fund Mozilla (and Safari to some extend) is to keep regulators happy. Being able to say "no no, it's not a monopoly, see!" is quite useful.
Idk if there is more data, but imo all you have to do is look at the financials, and it's pretty obvious that it's all about serving ads, billions of dollars in ads, directly or indirectly.