Most Helpful Guy
If the victim is a girl, it's usually done to rape her in a way she won't remember. If the victim is a guy, it's usually done to rob him in a way he won't remember. In both cases, the crime is extremely hard to uncover, which is why some people find it appealing. For example if someone spikes your drink with k. o. drops, takes you home (you will show no resistance whatsoever), rapes you and sends you away again, it's almost impossible for the police to find out who did it. Unless the culprit is a complete idiot and leaves DNA-traces on your body (namely sperm), there is no way to find him. You will probably wake up on the sidewalk somewhere with a headache and have no freaking idea what happened. Based on an abdominal pain and your unusual wake-up location you might deduce that you were raped but that's pretty much it. You won't remember a face or a voice, let alone a name or specific location. It's like a complete blackout.
I don't go to nightclubs but my best friend was once robbed this way. Someone spiked his drink and my friend didn't realize it. When he came back to consciousness on a couch in the bar-section of the club, everything was gone: his coat, his wallet, his keys... everything. It's really a terrible thing this exists.