Why do all of the Districs have to stay seperate? And why do the all have certian jobs?

That is the Capitol's way of showing the districts that the Capitol is totally in control. The Capitol gives and takes away. Just like the Games are there to show how they can just kill the kids, its the same thing.

The Districts all have different jobs because if each one did all of the jobs it would be less organized. Instead the Capitol could need coal so they easily just transport it from 12. They are probably all separate to keep rebellion from happening. If they were all together than a rebellion could easily happen. Also it depends where each District's resources are- 12: Coal
4:Fishing, etc. They would have to be in the area the resources are in.

I totally see why you asked...... I have no freakin' idea

Well, a district is sort of like a country so it has its own boundaries. Noone can go past it permission. To gain it's wealth, the districts have to invest in something, and that is their respectable roles. The Capitol is where the send their work off to, as the capitol leads them and everything.

By separating the resources, the Capitol makes it unnecessary (and impossible) for the Districts to communicate and therefore more difficult for them to revolt. The Capitol is putting it's eggs in separate baskets.