In battle, Parasect use their spores to put their opponents to sleep. This is a fairly powerful technique, but not a perfect one: most pokemon are light sleepers, and the excitement of a battle and pain of enemy attacks will awaken even a Snorlax.
Humans are not so immune to Parasect spores. Although the modern era has seen better sleep aids refined, insomniacs around the world have traditionally used pokemon in this capacity. And while sleep powder is common among all grass and bug pokemon, it is not especially useful; at times it does nothing, and typically these powders offer only enough sleep for a long nap.
Parasect spore is far more effective, and the problem with it typically falls on the other end: the especially deep sleep it produces usually involves oversleeping for an hour or two. At times, people have been known to miss entire days or even fall into years-long comas after inhaling too many spores. Despite these risks, Parasect spores were still dominant even in the cities of Kanto until this generation, and many older men and women continue to swear by them to this day.