e.g.: I am an amazon worker. I want to unionize. a trusted friend I knew before Amazon vouches for me to join the forum where we talk. Now, I can invite you, because I can vouch for you as a member.

If you turn out to be a strike-breaker spy, I might get booted with you, or maybe my reputation will be reset to zero, or I get to be watched more closely, etc.

1) https://en.wikipedia.org/wiki/Key_signing_party 2) https://en.wikipedia.org/wiki/Web_of_trust