For the worker it's actually great. It means that if the company wants to fire you they still need to pay your salary for at least the next three months.
Often times, especially in IT, when people are let go, it's often important for security reasons just to have them stop working immediately, and that worker is still going to get his salary.
When employees quit, it's not 3 months mandated, that's just the limit. You just work out your transition with your employer. Most job changes usually happen within just a few weeks.