Backups for your iPhone data are stored in iCloud, iTunes, or both depending on your preferences.
Use iCloud When:
• You want automatic backups to occur daily when your device is connected to both Wi-Fi and a power source. • You can’t connect to a computer in order to backup to iTunes • You want an automatic download that you can set and forget about it
Use iTunes When:
• You want to back up music, TV shows or videos that will not back up to iCloud • You can connect to a computer to sync from iPhone to your iTunes account • If you don’t want to use iCloud or want to make sure you have backups on site and via a networked backup system
Use Both iCloud and iTunes When:
• You want the additional assurance of more than one backup system • You want all your data backed up including purchases made through iTunes
Resource: Apple Support