Not always necessary on a stock bike, but can be very useful on a Superbike, if you run steep rake angle on the forks, there’s play in the suspension/bearings somewhere, the tyre isn’t quite true, etc.
I’ve occasionally had the situation without a damper fitted where the bike has hit a bump and gone into terminal oscillation, with no steering possible at all and just gone straight off. In this situation, backing off the power can help the bike re-settle. Fitting a damper helps stop the shake turning into a continuous oscillation in the first place, but won’t cure any fundamental set-up or build issues you may have.
The idea is get any needed steering damping as light as possible to avoid slowing the steering response down. You can also limit the fork rotation to restrict the amount of shake possible – about 15* either side of straight ahead is plenty.
There’s an article I posted on the newcomer’s FAQ section about head shake http://www.moto-5.com/index.php/topic,396.0.html , which goes into this area in more detail and has some useful follow-up discussion as well.