I think this is incorrect
To answer your initial question: what they actually do is coordinate the whole orchestra - it's not enough to start at the same time, they need to remain together throughout the piece; also they are responsible for the texture and dynamics of the performance - compare different recordings of the same work by different conducters and they can be quite contrasting.
Yes they are often self-important, dressed-up twats, but they are also highly talented, thoroughly practised, professional musicians themselves. It has been said (I forget who said it) that the orchestra is the ultimate instrument to play.
( tangledupinblue hairy badge with moving eyes, Fri 23 Sep 2011, 12:10, 1 reply)

To be fair...
...I was exaggerating. I know they are talented people, and having a non-playing coordinator for practising must be very beneficial. But at the actual performance stage I'm still completely at a loss as to what they actually do. Take the beginning of a piece for example the conductor waves his arms around for a seemingly random time/number of beats and then the orchestra all comes in together, see about 3:15 here:

www.youtube.com/watch?v=LnKJpYGCLsg

How do they know which of the conductor's varied flailings they should start on? How?! Similarly, througout the piece, none of his gesticulations relate to a beat within the music so how is he contributing to keeping them in time?
(flimflam_machine, Fri 23 Sep 2011, 12:26, closed)

I did suspect there was some tongue-in-cheek in your post
But in the example you give there, it looks to me as though at 3:15 he simply counts them in. How do they know what his varied flailings mean? Well, that performance isn't the first time they have worked with him - there will have been many months of rehearsals involving discussion about the performance as well as playing (and more than likely a lot of shouting from the conductor when members of the orchestra fail to understand his flailings).
He is not just keeping them in time but controlling their expression too. If you ever watch an orchestra without a conductor you will see they are shit. Individual sections may stay together, but the piece will quickly lose its coherence.
( tangledupinblue hairy badge with moving eyes, Fri 23 Sep 2011, 12:40, closed)