IncentiveRead it here.
The two main incentives are financial gain and spite, respectively, though the latter option has been used by angry employees who wanted to ruin a rival man's reputation. Attention seeking can be a factor, too.
VictimsLarge corporations have been targeted by opportunistic women. Any claim, regardless of how absurd, can take a long time to disprove. The negative publicity associated with a long, drawn-out court case can have a negative impact on the company's standing in the community. For example, a consumer might avoid dealing with the company until all doubt over their guilt has been determined. Not all members of the public accept a court's ruling when a company is cleared of wrongdoing, either. Due to this, quite a few companies have settled out of court because it was less costly than clearing their name. This is the reason opportunistic women have used this ploy.
Update: Ann Coulter's latest column shows how insane the "sexual harassment" charges were in the 1990's, with huge damages awarded to complaining women for the most absurd reasons. Read it here.
Update: Andrew McCarthy at the Corner also describes the hypocrisy and lack of substance in the charges against Cain, and the ambiguity and general stupidity of "sexual harassment" claims. Read it here.