I don't get why these APIs are so essential that Chromium loses its value. Why would a browser need an API in the first place? I get conveniences like bookmark

I don't get why these APIs are so essential that Chromium loses its value. Why would a browser need an API in the first place? I get conveniences like bookmark sync, and security features like avoiding malware domains, but one doesn't _need_ these to function, it's just somewhat more convenient or secure.