Page "Fishing reel" Paragraph 45

Properly set drag allows larger and more powerful fish to be safely landed, with the drag slipping below the breaking point of the line.

In combination with rod flex, drag will tire a fish by converting energy it expends into heat in the drag system.

Drag is set as high as possible without risking tearing the fish's mouth.

When the specified gauge for a given reel is used most will not be able to apply enough drag to break the line.