Originally published at: http://www.howtogeek.com/193055/what-is-a-custom-recovery-on-android-and-why-would-i-want-one/
Want to install a custom Android ROM — in other words, a third-party version of the Android operating system — like CyanogenMod? You’ll probably be instructed to install a custom recovery, too.
Note that the ADB backup mentioned will only help if you can boot your phone completely, where as you can restore a Nandroid backup anytime as long as you have access to the recovery.
Also, some Sony devices do not come with a recovery partition, so you need to flash a custom kernel or ROM with a recovery built in.