Insure a derelict cottage?

I bought a site with a derelict cottage a while ago. I now have planning permission and hope to demolish it and start building next spring.

A friend has advised me to insure the cottage (buildings cover only).

I have no idea why that should be necessary. If it burnt down tomorrow I couldn't care less. It has little value, is uninhabitable, and due to be demolished. There are no contents.

I have separate mortgage protection Insurance on the mortgage for the site.

Am I missing something?
Do I need to cover it?

Only issue I would see is public liability.

Yes, I thought that might be what he meant.
I'll investigate.
Thanks.