This is my understanding of the situation:
Before the mobile phone ban, prosecution of drivers holding a mobile phone while driving was based on the generic offences of driving without due care and attention, or dangerous driving, which was sometimes difficult to prove in court.
The legislation that came into force in the late 90s (from memory), made it a specific offence to make or receive calls using a handset that needed to be held 'for any part of the call" (meaning it was illegal to hold the phone to dial a number and then put it down while talking), as well as send or read text messages.
That meant that the police (or the CPS) now only needed to prove that the driver held the phone, and no longer needed to prove that it was distracting or dangerous to do that.
The issue that came to light as mobile phones developed over time, was that there did not seem to be a case against a driver who held the mobile phone, but wasn't actually making or receiving a call or texting (as can be demonstrated from the call log). That meant that using the phone as a camera or a satnav wasn't specifically banned, because there's no legislation forbidding drivers from using a camera while driving (which meant that any prosecution will have to fall back to the generic offence of driving without due care and attention). And, in essence the same applied also to a driver using his phone as a Nintendo game console while driving.
The new law is meant to fix that, by making it a specific offence to use your phone for any purpose other than those specifically permitted.
Again, this is my understanding.