Your iPhone (and iPad) automatically back up to iCloud by default, but local iTunes backups are still useful. You should create an iTunes backup when you’re switching to a new iPhone or installing iOS beta software on your current phone.
Local iTunes backups are more complete and faster to restore than iCloud backups. iCloud backups are still useful because they happen wirelessly so they’ll always be up-to-date, but iTunes backups are ideal for a full restore operation.
