The Federation wanted to harvest it to assist its general population and give themselves an advantage in their various wars and conflicts. They expected human lifespans to double with the harvested radiation. The Son'a claimed to want this resource to save their dying race, but revenge was closer to the truth.
Members of the Enterprise crew exposed to metaphasic particles began noticing the effects within days. Their appearance and behavior started reverting to more youthful stages. The most noticeable changes were Geordi Laforge growing new eyes and one of the crewmembers losing a baby as she reverted to a prior state of pregnancy.
Geordi developing functional eyes makes no sense. His blindness is genetic, not the result of a disease or injury, so no amount of healing would make his eyes work.