In theory one could imagine a scenario like a bank website refusing to be accessed unless the entire OS & browser stack pass attestation - as that would rule out things like keyloggers, malicious browser extensions, and session hijacking.
In practice it'll just be used to lock down content and force unskippable ads on users, of course.
I'm not interested in being hobbled for either of those problems. I remember when banks used to reject my browser because it wasn't IE in Windows. I remember when I had to look at webpages that were 50% advertising.
Screw that.
The important part is that "malicious" isn't up to you to decide anymore; if you have any "unapproved" software that acts in your interests and not others', this could theoretically be used to lock you out too.
Even that use case leads to bad outcomes. I already have to jump through hoops to get banking apps to run on my rooted phone. Banking websites refusing to run on anything but Chrome on Windows is a likely scenario here, and that's awful.
I don't want them to have a say in how I run my devices.
I'm also sure it'll end up with things like "your browser is too up-to-date" or crap like that.