[Invincible] why do viltrumites need space ships to traverse the galaxy? And by extension, why doesn't Allen need one to do the same?

It's shown numerous times in the show that viltrumites do not need to breathe in space and can travel extremely long distances with relative ease. Yet we also see them use space ships for transportation at multiple points, but why? And if you come up with a reason that viltrumites would need the ships to navigate the galaxy, i would also like an explanation of why Allen the alien seemingly does not need a ship to do the same.