The effect of this is that Let's Encrypt will only grant a signed TLS certificate if the request comes from one of my two accounts (authenticated with the corresponding private key). If the certificate request comes from a different account, no TLS certificate will be granted. This protects against man-in-the-middle attacks, specifically against attacks where someone between Let's Encrypt and my server would be trying to impersonate my server to obtain a signed TLS certificate.
Addendum: In case you're wondering where to get the accounturi value from, it can be found in your account file: