Plugins Newbie

I have installed nagios and it works fine.

Later i configured some services and it works ok, showing some results (for example disk usage, users login etc).

My problem is that after the installation of plugin in one server (NRPE etc), i get no difference in results. I see the same results.

So please help me:

  1. why i see results in the services menu without install plugins at all ??
  2. if i see the correct results without plugins, so what the use of plugins??

John