alias vs root in regex location (no replies)

Trying to serve nagios web GUI through nginx.

Part of config:
…
location ~ "^/nagios/(.+?\.php)(/.*)?$"
{
alias "/usr/share/nagios3/htdocs/";
try_files $1 $uri $uri/ /nagios/index.php;
#rewrite "^/nagios/(.+?\.php)(/.*)?$" /$1$2 break;

auth_basic "Authorization required to access Nagios";
auth_basic_user_file htpasswd;

# PHP-FPM Settings
include avi9526/php-fpm;
}
location ~* ^/cgi-bin/nagios3/(.+?\.cgi)$
{
alias "/usr/lib/cgi-bin/nagios3/";
try_files $1 $uri $uri/ =404;
#rewrite "^/cgi-bin/nagios3/(.+?\.cgi)$" /$1 break;

auth_basic "Authorization required to access Nagios";
auth_basic_user_file htpasswd;

fastcgi_param AUTH_USER $remote_user;
fastcgi_param REMOTE_USER $remote_user;

include avi9526/fcgiwrap;
}

it's seems to be working. If i change "alias" to "root" and uncomment "rewrite" directive for both php and cgi scripts it will work as well. So, what the difference? And why most online manuals suggests to use "root" followed by rewrite?