Trusted Networks: what types of threats we are protecting from?

[petRUShka]

README says:

This helps to avoid leaking personal information on untrusted networks by ensuring that certain network tasks are not running in the background. Currently, this is used for mail syncing (see the section below on Syncing and Scheduling Mail), Tarsnap backups (see the section below on Scheduling Tarsnap), BitlBee (see the section below on BitlBee), and git-annex (see the section below on git-annex).

I'm wondering what kind of threats are on the table in case of for example Syncthing? It is protected by cryptography. What information could be leaked?

Another question: what conditions should be satisfied by a network to considered network trusted enough? How to separate trusted networks from untrusted? By what criteria?