Whispers in the Wind
At first glance, the data looks pretty innocuous. Who cares if people know what browser you're using, or what plugins you've installed? Is it the end of the world if they know that you're a Mac user?
Well, individually, the data is indeed innocuous. The original purpose in giving site owners this information is so that they can adapt their application to better suit the device it's run on. My homepage animations, for instance, adapt to the size of the screen and whether it's run on a mobile or desktop device.
United, I am. Divided, Unknown.
Once you put all of the information together, however, there's a potential for misuse. It's very easy to be the only person on the digital planet running that exact version of browser on that exact OS with those exact plugins/fonts installed, et cetera.
Using this information, it's possible to derive a Unique User Identification Number and use it to track you across the internet, regardless of where you're connecting from.
An advertisement network will check if they have this identification number. If they do, then that means that they already have a list of sites the user has previously visited, along with any other data they've determined about the user. This is then used to display ads relevant to your assumed interests.
This type of invasive tracking, on the other hand, is also used to flag suspicious activity with your online accounts. I'm sure you've encountered sites that require you to click a link in your email to verify that a new device was indeed you. This is a great and highly effective service to thwart malicious login attempts.
But it's hard to know what goes on behind the curtains. Those sites providing these security services could also be using that same personal information in many other ways. The EU's GDPR is supposed to ensure companies are transparent about what's done with such information, but it's difficult to enforce and easy to evade.