The Spacing Guild plays a pivotal role in Dune media. Their monopoly on space travel makes them a rich and powerful part of the Known Universe.
They use so-called Navigators as pilots for their starships who use prescience to "fold space", an ability which transports ships over massive distances.
To gain these abilities the Navigators depend heavily on the substance Spice, also known as Melange. This drug, while beneficial, has the major drawbacks that it is addictive and that withdrawal is lethal.
A Navigator needs to be kept in a special tank containing gaseous Spice at all times. This severe exposure mutates the Navigator over a period of time up to the point that the person no longer looks fully human.
Scytale looked at the Guild envoy. Edric swam in a container of orange gas only a few paces away. His container sat in the center of the transparent dome which the Bene Gesserit had built for this meeting. The Guildsman was an elongated figure, vaguely humanoid with finned feet and hugely fanned membranous hands—a fish in a strange sea. His tank's vents emitted a pale orange cloud rich with the smell of the geriatric spice, melange.
From Dune Messiah - 1969
The question: A Navigator sacrifices a lot to become one, but what do they gain from it?