Firefox

It prevents apps from opening links in your browser directly, since they have to go through URLCheck first. Let's say you click a link in your email, and instead it opens a "google.com/url?q=https://amazon.com" or a "safelinks.outlook.com/?url=..." instead of just taking you to Amazon.com. URLCheck will get rid of the unnecessary redirection and allow you to go directly to the site.

Adding onto that, my pet peeve with email links is them showing you a link that says Amazon.com, but then you go to click the link it opens a bunch of email tracking links before finally taking you to Amazon.com. With URLCheck you can actually stop these links from opening, and go to the website directly in the browser yourself.

If you're familiar with that issue that popped up regarding ".zip" domain names, and how they can be engineered to look like an official URL, this is a non issue as you'll get a warning if any link contains malicious unicode characters that could be mistaken for something else

Also, if you have multiple apps that can handle a link (maybe different youtube apps like libretube, newpipe, grayjay), you can pick which one to open after clicking a link. Android does have a stock app picker, but it's very easy to mistakenly set an app as default.

Android apps can also track what apps triggered them to open - URLCheck can mask this, and even set some advanced flags for how the link handler app should be opened.

To be honest after using it for a while, I really wish there was something similar available for desktop