Why is the backup so important?

Because your data exists only on your phone. The Fenzly server knows neither your places, nor your circles, nor your messages: it cannot give anything back if the phone is lost, broken or replaced.

The backup is encrypted before it leaves the device. Two modes exist: the automatic backup, whose key is split into two halves that neither Fenzly nor your cloud space can read alone, and the Fenzly-hosted backup, protected by a recovery code only you hold.

Turn it on from day one in Settings → Backup, then check the date of the last backup from time to time.