Hydrus Four is the only planet on which the miracle restorative Nucleon can be found. Now, this WOULD be plenty of cause to journey to the planet except for a few middlesome problems.
Specifically, the fact that the planet is inhabited by creepy obsessive zombie mechanoids who guard the Nucleon. Also: the only inhabitants of the planet who are NOT Robo-Zombies live only to warn people of the zombies.
Hydrus Four is well known for the high quality of its medical professionals, so presumably there is more to this planet than has been seen.