Why did the Voyager probes use gravity assists to reach the outer planets?

Voyager 1 and Voyager 2 relied on a rare alignment of the outer planets to swing from one world to the next. In a gravity assist, a spacecraft passes behind a planet relative to its motion and gains speed from the planet’s orbital energy, while the planet loses an immeasurably tiny amount. This allowed the Voyager probes, launched in 1977, to visit Jupiter, Saturn, and in Voyager 2’s case Uranus and Neptune. Without such maneuvers, sending one mission across the outer solar system would have required much more propellant.