Having caps lock on your mobile device is not a daily requirement, but on occasion IT CAN BE QUITE USEFUL.
Trying to add all caps in a sentence on the iPhone can be a tedious experience by clicking the shift key each time you wish to add a capital letter. But, Apple has built-in the caps lock function on their iOS keyboard and it just needs to be enabled.
Open Settings on your device and browse to General
From General, choose Keyboard
In Keyboard settings, enable the Caps Lock function.
Now that it’s been enabled, simply double tap on the shift button and you will see the color change to dark. When you’re done, touch the shift key again to disable caps lock.
OS X Tips and Tricks 
Well written.