Pain enhancers do the opposite of painkillers: they reinforce the sensation of pain rather than alleviate it. This shouldn't mean that when you have taken them you would be walking around in constant pain, you'd just notice a sharper jolt of pain when you accidentally cut yourself, stub your toe, et
I see three advantages for this.
1. The pain normally involved in minor injuries has something of a "canary in a coal mine" function: if you, say, lightly tap the business end of a meat grinder with the tip of your finger, the cut will hurt a little, and you will know not to do that again, but more importantly, you will know not to shove your whole arm in there. In some situations, such as those where even an otherwise unnoticeably small cut might result in a highly dangerous infection, enhancing this functionality of pain could be life-saving.
2. Subjecting yourself to (potential) pain is widely known to render you as well 'ard and therefore desirable to (some members of) the attractive sex.
3. One of the risks of taking painkillers is becoming addicted and/or tolerant to the drug, resulting in a need for increased dosage to achieve the same effect, and even having to take medication in order to reach a "neutral" state. Allowing yourself to become tolerant to pain enhancers should therefore yield an "instant painkiller" effect that can be summoned simply by quitting the pain enhancers briefly.
These pain enhancers are mostly concerned with nociceptive pain. There might also be applications for other forms of pain, such as chronic, though I currently can't think of any non-masochistic grounds for taking chronic pain enhancers. Actually, for masochists, any kind of pain enhancers would probably work well, so add that as a fourth advantage.
Edit: tormented artists who've run out of ideas might be served by psychological pain enhancers, but maybe this is a different idea altogether.