You can gift or sell them and people can display them on their profile. Could show that you supported some cause, etc.
Discussion
Where can we get them?
People are awarding them to users on https://badges.page and https://nostrbadges.com Anyone can create them and award them. It's up to the user to accept it.
I have to give my key to get them?
I dunno about that.
you'd need to use a browser with an extension like Alby or nos2x. these extensions act as a signing device and hold your private key so you don't have to enter it into the website.
Will it lead to a situation where certain notes/features are visible or accessible only to people with certain badges? That's be a shame
How to rely on the “badges.page” site? While we should enter our private key?
It tastes like a “Manipulator-in-The-Middle”. I will recommend NIP-07!