Why does the HTML directory need to be writable?

I set up pi-hole with an existing Nginx service according to the documentation. The guide says the HTML directory needs to be writable:

  1. Make sure the html directory is writable
    chmod -R 755 /var/www/html

Anyone know why /var/www/html needs to be writable? I didn't find any file there that seems should be written at run time.

I remember PHP writing a temporary file to remember until when a temporary disabled/enabled state is running. No guarantees this is still the case (I think it is not).