Quite simply, if an accident or incident occurs it's the difference between having one safety trained person; the Driver, or two. Consider that the Driver is usually the first person to feel the impact in a collision; consider that the Driver has many things to attend to in the event of any emergency that don't in any way concern dealing with the passengers; consider that the Driver may be incapacitated not only through a collision and injury but also through shock or ill health.
The Great Western network has in recent history suffered some very nasty accidents. We had Southall, Ladbroke Grove and Ufton Nervet. We came very seriously close to having another one very recently, thanks to a certain heritage operator - make no mistake, in that incident the marvellous foolproof safety systems that make crashes impossible were thoroughly overidden; had that train passed the signal just minutes earlier we would have seen a major train crash. We had the shocking incident at Lavington in which a tree smashed through and demolished the cab of a speeding HST; only by sheer luck did the Driver walk away. A few weeks ago we suffered a fatality in which the victim smashed through the windscreen and impacted the cab bulkhead directly behind where the Driver's head would have been, had he not been quick enough to dive onto the floor. Transpenine Express had a Driver suffer a heart attack in the middle of nowhere (as seen on TV). Virgin recently had a Driver electrocuted by 25,000 volts of traction current..."Sh*t happens", as they say, and when it does you're a hell of a lot better off when there's decent, competent help available. Those who like to imply that the railway stopped having accidents and incidents when BR ceased to be are living in a dream world.
Put it this way - when I did my Guard training and we covered the bits about accidents and the emergency procedures concerning them, the trainer was very clear about the 'golden rule' - the procedure comes first, the passengers come last. That applies to all of us, and if the Driver doesn't make it your Guard will do the neccessary. If the Driver is still breathing and able to do his bit, you have a clued-up second person who can assist you. If you have only a Driver, whether he makes it or not he isn't going to be helping his passengers. Your friendly Ticket Examiner/customer service bod/whatever will be considerably less helpful in either case.
So, I hope that helps to clear it up for you!