A single train only needs one driver, not eight.

Eight trains would need a gap between them ­— typically several km per train — so the capacity of the line is increased by having longer trains.

Air resistance would be higher for eight trains.

Those 8 locomotives can be controlled by a single team of people. Additionally it allows greater freight throughput because you need to maintain minimum distances between trains. It's actually extremely uncommon where I live to see a train with a single locomotive.

Well technically that specific train was a stunt (at the time BHP Billiton's regular run used 336 cars and 6~8 locomotives, now 264 cars and 4 locomotives), other commenters have answered as to why you'd want to run longer trains (and AFAIK modern trains rarely run a single locomotive these days, a pair is just more efficient and convenient even on passenger trains)

See https://en.wikipedia.org/wiki/Distributed_power for more information on running distributed power.

Not to mention, if a loco had a failure the second loco would get the train off the main or to a convenient yard swapping out the failed locomotive.