It is the same link protocol as http: or ftp: or mailto: or tel: or magnet:, although very less recognised. Official Bitcoin github MediaWiki Link
Localbitcoins.com offer its registered users to associate itself(site) to bitcoin: protocol, when user is signed in, and thus he clicks a link with bitcoin protocol, localbitcoins.com will automatically fill in the receiving address and amount text boxes with the values from link.
Yes, it just sends what it displays, address(required), amount and/or message (both optional)
On webpages, anytime it is marked down in html as
<a href...> tag, it will be a clickable link.
There is no other use of it when it is not clickable, other than that you could see receiving address and amount, same as if you see
mailto:email@example.com and you could just deduce the website address or email address.