Surely you’ve heard of OAuth.

But if that’s rhetorical/curt, then the answer can also be found on the home page of the site: it’s a service with a paid component and the login is to associate the purchase with your account. Very common across the web.

That explains signing in to pay for the service or to access the paid tier options, but why do you need to sign in before that?

So they can track their users? The free tier users are still users.

You don't go to

https://ffmpeg.guide/demo you can use the free version without signing in