Why is the main folder required to be on a volume, for updates?

I have an existing nextcloud installation, that I want to migrate to nextcloud:fpm docker container. To this end, I am considering the following docker run command, that allows me to reuse my existing mysql db and data folders:

docker run -d \
-v /nextcloud:/var/www/html \
-v /nextcloud/apps:/var/www/html/custom_apps \
-v /nextcloud/config:/var/www/html/config \
-v /nextcloud/data:/var/www/html/data \
--net="host" \
-e MYSQL_DATABASE=<mysql_db> \
-e MYSQL_USER=<mysql_user> \
-e MYSQL_PASSWORD=<mysql_pass> \
-e MYSQL_HOST=127.0.0.1 \
nextcloud:fpm

My question is: in the image readme is stated that mapping /var/www/html is required for updating, but I do not understand why: If the container will be thrown away when there is an update, and a new one will be pulled, I think I do not need that mapped volume... right?

Thank you!

Start with the image README section that says /var/www/html must be mapped for updates, then compare it with the posted docker run command and its separate apps, config, and data mounts. Clarify the role of the main volume during container updates and update the README wording so the required persistence and migration behavior are unambiguous.