Carmine Gallo in HBR:
According to molecular biologist John Medina of the University of Washington School of Medicine, the human brain craves meaning before details. When a listener doesn’t understand the overarching idea being presented in a pitch, they have a hard time digesting the information. A logline will help you paint the big picture for your audience.
In Hollywood cinema, one of the greatest loglines of all time belongs to the iconic thriller that kept kids out of the ocean during the summer of 1975:
A police chief, with a phobia for open water, battles a gigantic shark with an appetite for swimmers and boat captains, in spite of a greedy town council who demands that the beach stay open.
What makes it work?
The logline for Jaws identifies the key elements of the story: the hero, his weakness, his conflict, and the hurdles he must overcome — all in one sentence. It depicts the overarching storyline in an interesting, straightforward way, rather than focusing on details that might seem meaningless without the context of the bigger picture.
More here.