Here is what I understood. I m expecting somebody else to correct me if I understand wrong.
for small site (if you don't have many hosts to monitor) just use active check would be fine.
nagios server (nagios/web interface/check_nrpe/plugins)
host1 host2 (nrpe/plugins)
for large amount of hosts / different locations, it needs passive check from distributed nagios server to central nagios server. However, distributed nagios server would still use active check for the hosts it responsible for checking.
central nagios server
distributed nagios srv1 distributed nagios srv2
/ \ / \
host1 host2 host3 host4
above is just my 2 cents, may not be correct, trying to understand it too.