Ads. To be precise this on it’s own provides a way for servers to be certain of the environment the pages run (browser, plugins, os). Protecting ads or other functions come from servers refusing unattested configurations or configurations they don’t like (i.e. running adblock, running firefox, running linux).
It should be noted that “being certain of the environment the pages run” requires controlling the client software being executed which requires preventing the user from modifying said executable which requires the browser to either be closed source or, more effectively, controlling the user’s hardware via blackbox verification chips (e.g. TPM DRM). It’s not just advertisers that would benefit but any website that wants to DRM content.