Signing bonuses often have a requirement to stay for a certain amount of time (or you have to return it).

Some signing bonuses help compensate for losing bonuses or unvested stock at a company you're leaving.

Signing bonuses are also great from a company's perspective, because they're a one-time payment that may get you to accept a lower salary. From the employee's perspective, they may be good or bad depending on your needs; consider carefully if you'd rather negotiate for a higher salary instead.