What is the utility of the italic angle? I know the app it uses for some functions, but this value is needed for Design Apps or OS?
I searched in the forum but I didn’t find any info, if this thread is duplicated, please delete it 
That depends on the app. Some might use it to display a slanted cursor. Others to sort styles in the font menu. All that is not officially documented.
Why do you like to know?
If you export Regular and Italic fonts with the same weight class 400 and italic angle 0, sometimes the styles are sorted Italic, Regular in Adobe apps. Adding a value to italic angle helps to sort the styles properly: Reguar, Italic.
The Cocoa Text Engine uses it to determine if it is to be treated as an Italic or not. Careful when your upright is slanted very slightly (e.g. 1°), you may want to set it back to zero with a custom parameter.