Mostly to satisfy the TV director and public's demands for loads of camera angles and up to date race information. Up in the mountains is where you want more cameras on bikes as there will be more groups forming and it might not be possible to have a helicopter flying because of cloud/weather conditions.The moto into which Porte crashed was itself held up by two or three other motorbikes in front of it. WTF do they need so many of them on the race for?
A few for neutral service wheels (you can see one checking if Froome just needs a wheel).
A few for race information/timing.
Police escorts at front and back and crowd control (although not much crowd control seemed to be happening today).
It's tricky though when the roads get narrow there isn't really anywhere for the motorbikes to pull over especially when the crowds start to encroach. I'm sure questions will be asked of the organisers although suspect they will probably bury their heads in the sand in true French fashion.