[BrandoElFollito]

This is supposed to protect both the employee and the employer.

The employee to give him some time to find a new job (he is entitled to some days off during these 3 months to interview).

The employer to (at least theoretically) force the employee to still work to prepare a handover, docs, etc.

What happens in practice depends on the case : these 3 months may be shortened via a mutual agreement, the employer may force the employee not to come to work (but still pay him) and other arrangements are possible.