When you backup your iOS devices using iTunes on OS X, the backup files are stored in the boot volume Macintosh HD. Unlike your media files in the iTunes library, there is no way to instruct iTunes to store these files in a different volume.
Depending on the number of devices that you have and the number of times of backup, these backup files could occupy few GB to easily over 10GB of hard disk space. If your Mac does not have a large boot volume, as do most Macs with SSD hard disk, it pays to remove those unwanted backup to free up precious hard disk space. You might even have backups of iPhones that you had already sold.
To check how much space is taken up by iTunes device backups, select from Finder menu Go > Go to Folder (or Command-Shift-G) and enter “~/Library/Application Support/MobileSync”. Select the Backup folder underneath and press Command-I to get folder info.
To remove old backups, go to iTunes > Preferences and “Devices” tab. Select one or more of the unwanted backups and click the “Delete Backup” button to remove them.