The nginx resolver directive is required because the system resolver blocks. Nginx is a multiplexing server (many connections in one OS process), so each call of system resolver will stop processing all connections till the resolver answer is received. That’s why Nginx implemented its own internal non-blocking resolver.
If your config file has static DNS names (not generated), and you do not care about track IP changes without nginx reload, you don’t need nginx’s resolver. In this case all DNS names will be resolved on startup.
Nginx’s resolver should be used, if you want to resolve domain name in runtime without nginx reload.