Docker: NGINX and PHP: Why change the default user/group?

I started learning about Docker and setting up NGINX and PHP. I looked at some examples and saw the following example: https://github.com/aschmelyun/lc-the-docker-tutorial/blob/main/nginx.dockerfile

There are two dockerfiles, one for NGINX (nginx.dockerfile):

FROM nginx:stable-alpine

ENV NGINXUSER=laravel
ENV NGINXGROUP=laravel

RUN mkdir -p /var/www/html/public

ADD nginx/default.conf /etc/nginx/conf.d/default.conf

RUN sed -i "s/user www-data/user ${NGINXUSER}/g" /etc/nginx/nginx.conf

RUN adduser -g ${NGINXGROUP} -s /bin/sh -D ${NGINXUSER}

and one for PHP (php.dockerfile):

FROM php:8-fpm-alpine

ENV PHPGROUP=laravel
ENV PHPUSER=laravel

RUN adduser -g ${PHPGROUP} -s /bin/sh -D ${PHPUSER}

RUN sed -i "s/user = www-data/user = ${PHPUSER}/g" /usr/local/etc/php-fpm.d/www.conf
RUN sed -i "s/group = www-data/group = ${PHPGROUP}/g" /usr/local/etc/php-fpm.d/www.conf

RUN mkdir -p /var/www/html/public

RUN docker-php-ext-install pdo pdo_mysql

CMD ["php-fpm", "-y", "/usr/local/etc/php-fpm.conf", "-R"]

In both cases new user & group are created (user laravel assigned to the group laravel), and then using sed command config files are modified:

• For NGINX it replaces all occurrences of the user www-data with laravel in /etc/nginx/nginx.conf;
• For PHP it does a similar thing: It replaces occurrences of user & group www-data with laravel.

3 interrelated questions:

Question 1: Does this mean that NGINX and PHP (I guess just FPM, and CLI will not be there because it pulls php:8-fpm-alpine base image from Docker Hub which doesn’t include CLI, right?) will run as the laravel user inside containers? (If yes, please also look at Question 3).

Question 2: Is there any practical reason to change the default user for NGINX and PHP-FPM, as in the examples above?

Question 3: In php.dockerfile at the end there’s this command:

CMD ["php-fpm", "-y", "/usr/local/etc/php-fpm.conf", "-R"]

I searched on Stackoverflow what this -R means, and I found the following answer: https://stackoverflow.com/a/49178410/4437206
If I understand correctly, this means that PHP-FPM will run as root? If so, then it will not run as the laravel user we added and therefore, what’s the point of adding the laravel user?