Thanks for that.
So.........what happens if the designated/proposed leader is having a crap/off day ?
I assume they already have a 2nd rider already nominated ??
Like most races, a Grand Tour (so the Tour de France for instance) has a lot going on in it and not every team is there to do the same thing. Some have a good contender to get on the podium at the end of the race and they are going for GC (general classification I think) what matters to them a lot (sometimes to the exception of everything else) is where their GC rider ends up on the last day of the event, or with the TdF the day before because it's now traditional to use the final day as kind of a big parade. So the GC rider isn't supposed to have off days, if they have one they are supposed to make it as 'on' as possible and whatever time they lose they are expected to make up in the days to come. A team with a GC contender who can't do this, doesn't really have a contender. Often, but not always, because of the way some other team members will be favoured helpers you get a second rider close to the top who could take over in the case of problems (Froome for Wiggins in 2012 for instance) but for all the talk of 'plan B' if the rider isn't up there and good enough from the beginning they probably aren't going to do the job. The requirements to be a GC contender mean that they are rare and two true GC contenders in one team rarely works out (hence no Wiggins in 2013 & 14) in theory a GC contender who isn't the main choice should make an excellent super domestique (domestiques are there to ride for the winners, not to win the whole thing themselves) but it takes a certain mindset to be a GC contender and it rarely gels with also being a helper once you've reached the top, although sometimes they do the odd day of it.
Aside from people going for GC, some are going for the other jerseys (Points - Green, kind of best sprinter but not really of late, Mountains - polkadot, best climber, Best young rider - white, best rider under 26, showing how important maturity is in cycle racing.) which means they will be riding with the leaders of those jerseys in mind and not the overall winners, especially as often the points of the race that decide Green and Polkadot aren't at the end point of the race.
There is also a best team classification which isn't always won by the team with the best rider (look for riders with yellow numbers on their jerseys).
Finally, each stage has a winner, it has a most combative rider, and it has riders that will be in breakaways. All of those get some limelight which means advertising for the sponsors, so a team might have no chance at winning anything, but its riders make the breakaways a lot so get plenty of TV time and that's enough of a 'win' for them.