yes, edited, thanks
To check the route for errors in the instructions (which are more common than most people are prepared to admit);
To check the distance readings on the computers used to set the course (to ensure some semblance of accuracy); and
To check road conditions, including signposting, surface and traffic flows.
There is a duty of care that lays with organisers to ensure that all those facets of the event are taken care of before the participants set off.
I agree that scouting by bike can be better than by car, for other reasons mentioned in this thread though.
I think what may help, most specifically with cue sheet accuracy, is having someone otherwise unfamiliar with the course do the car scouting / bike pre-ride. Organizers who are familiar with much of the course already are sometimes apt to miss bad cues, just simply because their brain inserted the correct information (instead of what the sheet said) when they arrived at the turn.
Just speaking for myself, it means a lot when my RBA has done the route on his bike. It means that he has experienced much of what we will, and "eats his own cooking" for lack of a better term.
Even though an RBA is under no obligation to ride their routes via bike, I would have a much lower opinion of him if he just hopped in his minivan to scout it out and said "good luck!" on the day of the event.
After riding the whole route within the time limit (and you can do it with a small group of people as a test ride), it is a good idea to ride various sections of the route at different times of the day, week, and year. Become very familiar with what the riders may see and experience. And send other riders out on parts of the route too.
A 200K might not require so much attention, but the 1000 and 1200Ks should, especially if they might attract people from all over the world.
One of the most important points that can be discovered on a recce ride is the position of the sun at sunrise and sunset. The worst condition for a rider is with the sun straight ahead, meaning motorised traffic coming up from behind can be blinded and not see the rider. Observing for these conditions may take a number of hours depending on the ride window through the location.
A recce ride also can determine whether turns across busy highways are appropriate, particularly late in an event when a rider's judgment might be just askew enough to take a risk that could end badly.
Organising a long ride like a 1000 is not an easy task and cannot be done effectively from the driver's seat of a car or in front of a computer screen.
And the point that has already been made about organisers failing to include vital instructions "because everyone knows what to do there" cannot be reinforced strongly enough, and especially if there is a chance even on person from outside the organiser's region might participate. Having critical and independent eyes following draft route instructions is more than helpful is determining mistakes and misunderstandings.
I know of some incredibly well-planned rides that have been scouted mostly by car. People that aren't thorough are going to do a crappy job if they ride a route 5 times
One thing I notice is that bikes are sensitive to little bumps, cars are sensitive to big bumps, so you can have roads that seem fairly smooth in a car that are rough on a bike, and vice versa.
Your perception of the volume of auto traffic is different in a car. You'll meet more oncoming vehicles, but see very few going your way, as compared to riding a bike.
Some of the best routes I've ridden have been scouted, designed ... and ridden by a particular person in the BC Randonneurs and another particular person in the Victoria group of Audax Australia. Both people ride a lot ... top of their clubs for events and distances ridden. They know what works for cyclists and what doesn't, and they've designed routes that not only look good on paper, but which they themselves enjoy riding ... and that enthusiasm spills over onto the participants.
One of the things I'm looking forward to when we return to Victoria, is doing some of those rides again.
I'm going to ride with you.
...and, how much time in advance, I must announce date of the ride?