Much of it is a marketing gimmick by the drug companies. The other part is that they cover up the symptoms. You get temporary relief, but not a cure. You’ll get better faster if you just drink more water and sleep more. This is especially true for colds and flus.
I was reading comments on a post about a change in DayQuil/NyQuil’s formula. This is particularly relevant because my roommate is sick. He’s taking lots of NyQuil, though. I haven’t had the certainty or courage to tell him not to, though. I think he sleeps enough, although he could probably drink more.
As far as the comments on the linked post, I agree with those who agree that cold medicine doesn’t help you get better. It suppresses or covers up the symptoms. This is also what my parents have told me, and it seems to be true. I’m not sick now, but if I do get sick, hopefully I’ll remember this.
A lot of people are getting sick around finals time, and a lot of them are taking medicine. Often a bad idea. And it’s true: they seem to stay sick for a while.