Freight trains are often exceptionally long because of the limited density of rail networks in many areas. Once they reach distribution centers, the freight will often make shorter trips on trucks or other trains to its final destinations.
In rural areas, long trains are often more common as local traffic is less dense.
There is some debate over the safety and necessity of 200+ car trains spurred in part by non-industry frustration with the practice.
I will not comment on this.