User's privilegies (Mysql, Nginx, Php-fpm)

Question

got questions of unsuccessful searches

1 - Why almost every software should run with its own user, is so simple run everything with a single user, why no one do that ?

2 - Why can't I just set the home dir to "/dev/null" and the shell login to "/sbin/nologin" for everyone ?

mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash # Mysql
nginx:x:493:489:Nginx user:/var/lib/nginx:/bin/false # Nginx
nobody:x:99:99:Nobody:/:/sbin/nologin # Php-fpm

Why so many alternatives, why its so confused, I can't understand, WHY, WHY, WHY ?

Thanks O.o

Answer 1

1 - it's for security. f.e. if apache runs as its own user and someone hacks the apache, he could not controle mysql and he could not read others dirs, because the apache user has only rights for apache and it's directories.

2 - mysql is running as user "mysql". some daemons, like for mysql, need to start other programms or daemons etc. and for this it needs a valid shell. and mysql have to read the files in the home dir also. it's all about security :)