Simple a rider's noble phantasm is usually their sweet ass ride. A voyager's Noble Phantasm is their actual journey. A common theme in myth and legend is referred to the hero's journey. Mythologist Joseph Campbell wrote about the heroes journey and it's structure. Many times in myth it's not a single deed but what the hero's journey represents. If you need an example just look at the Epic of Gilgamesh as a whole, specifically examining how Gilgamesh transformed from an arrogant hero to a wise king by the end.
Here I have some examples of possible Voyager servants
Example1: Do you agree that Marco Polo is eligible to be a servant? I do. In the create a servant topic people have made him both as an Archer and a Rider. Now i'm not saying that either of these classes don't make sense. But when you think of Marco Polo many people simply think of his journey to China and back more than any one thing he might have done. In this sense his Noble Phantasm is the journey itself, probably manifesting as a Reality Marble.
Example2: Do you agree that Neil Armstrong is eligible to be a servant? He's been mentioned and made a brief appearance in Fate/Extra Last Encore and got name dropped in Fate/Requiem. Once again he could be a rider because of his spaceship, but what people know him for is the journey to the moon and his famous line, "One small step for man....". Once again he could be a servant who is about their journey more than anyone thing they did or any vehicle they rode on.
Lastly I also want to point out that this topic shouldn't become about Joseph Campbell and the details of this works. The idea is that for some servants the Journey is more important than the actual deeds.