What browser is visiting the page?

Functionally useless. With the web standardized, we shouldn't need user agents anyway. It would be more beneficial to ask "do you support X, Y, and Z?"
- It's called feature detection and it goes a long way back, even before Modernizr popularized it.
  
  Popularized? That gets less than 100k downloads a week
- That's exactly what you're supposed to do with the modern web, via feature detection and client hints.
  The user agent in Chrome (and I think Firefox too) is "frozen" now, meaning it no longer receives any major updates.
- Youtube currently (for weeks now) does not work on Firefox, if you don't use a Firefox user agent. Google doing sketchy things again.
  
  I’ve not run into this issue and use Firefox exclusively with ublock origin
  
  YouTube works fine on Firefox…
  
  It works fine?
  
  Uh... I use librewolf that force a chrome + windows user agent and its totally fine?
  
  again? Did they stop anytime recently?
  
  ? I just tested and it worked fine.
- User agents are useful for checking if the request was made by a (legitimate self-identifying) bot, such as Googlebot.
  It could also be used in some specific scenarios where you control the client and want to easily identify your client traffic in request logs.
  Or maybe you offer a download on your site and you want to reorder your list to highlight the most likely correct binary for the platform in the user agent.
  There are plenty of reasonable uses for user agent that have nothing to do with feature detection.
  
  Aren't user agents just a plain text header? Couldn't a malicious agent just spoof a legitimate one?
- Web UI for touch screens is a lot different than keyboard and mouse. I still switch to desktop most of the time because the mobile site will lack critical info, though. They "have" to streamline the experience for mobile, but I hate it when they fully remove features.
- You have to use user agents to fool scummy websites into thinking that you're using chrome or edge.
- Lazy web developers or clueless managers have entered the chat
- I know Safari 15.3 doesn’t support feature Y, but I also know the current version does. Now I want to know if I can just use the feature or if I need to program around Safari 15.3. It would be nice to just look at the server logs from last month and see if someone still uses it.

User agents are essentially deprecated and are going to become less and less useful over time. The replacement is either client hints or feature detection, depending on what you're using it for.

"yer a jedi, harry" - Gandalf
- Listen here, Gandalf, you fat oaf! I'm not a fucking Jedi!

Is it... (scrolls wheel of browsers) Lynx?
- I'm still amazed at how usable Lynx is, given the insane premise of the application.
  
  What's so insane about it? Web browsers are an evolution of the old gopher protocol. All this stuff has roots in text consoles.

A URL is not an agent string, just saying.

I want a WordPress plugin that refuses to load my site for anyyhing newer than Netscape 3 and pops up a modal "you need to upgrade your browser" pane.

A new browser touches the beacon

There are some use cases other than web page compatibility. One for me is in dealing with firewall and proxy policy, if the agent is a browser and comes in on specified explicit ports then force authentication, things of that nature.