Because if there was one instance of it during trial or there was someone that had it while on the medication even unrelated it's probably in there.

So say someone took Advil then had, as a minor example, violent giggling fits completely unrelated. Now violent giggling fits is going to be listed as an adverse effect for Advil because, maybe it caused it?

Now replace violent giggling with fun things like: stroke, Stevens-Johnson syndrome, or whatever other fun things are listed on Advil...

Basically, likely to scare yourself for no reason. But for more serious drugs? Maybe worth some digging.