Your lenses and frames can be expensive. It is better to insure them and safeguard your financial interests with spectacle insurance policy.
Yes, it is mandatory in case of theft or burglary. In case of damage, it is not.
You should do so the moment you buy them.
Yes, it covers your lenses as long as the power of the lenses before and after the damage is not different. The ownership of proving the same lies with the customer