セキュリティ キーの目的は何ですか?

When you create a tunnel in admin.atlassian.com, you always generate a unique security key that needs to be added to your Data Center instance to establish the tunnel connection.

A security key is unique to every tunnel. It ensures that only a trusted client (Data Center side) can connect to the tunnel server (Cloud side). This is to prevent any kind of Man in the Middle attacks.