The primary purpose of an IDP is to provide a translation of the privileges granted by your driver licence into numerous other languages. Because the UK and USA share the same language (well, at least, when it is in written form, anyway) the IDP is of little value to you in the USA.
A secondary purpose of the IDP is to identify exactly what vehicles you are entitled to drive (cars, motorcycles, trucks of various sizes, buses, etc.). But, if that is self-evident on your home driving licence, and your home licence is in the same language that the cops read in the place you are visiting, there ain't much point to having one.
I have been buying IDPs for 20 years (my company pays the expense). I've only ever needed to show the IDP (for translation purposes) twice, once in Angola, and once in Russia.
Michael