Things I don't understand: pharmacopoeia

If one medication can treat a vast array of conditions, you'd expect that those conditions would have something in common, right? Maybe they attack the same systems in the body, or respond to a set of active ingredients in a similar way, right? Makes sense.

I've got this new medication, I take it every night before bed and it is supposed to prevent migraines. Reading carefully the provided information from the pharmacy ("looking for goofy side-effects"), I come across the list of other common applications for this particular medicine and promptly pulled a screaming u-turn in the parking lot, ran into the pharmacy, and assured the nice woman at the desk that I am not, honest to god, a bed-wetter. Nor am I suffering from an addiction disorder, an eating disorder, or an obsessive-compulsive disorder. Neither do I have anxiety nor panic attacks--at least, not until I read this list. Also premature ejaculation isn't so much a problem for me.

Okay, seriously, now. Biomedically speaking, what unites bed-wetting and OCD? What common ground is shared by eating disorders and panic attacks? Where does this all unite with migraines? Because if they all react to this particular medicine, there must be a reason, right?