What is the OpManager Agent?

What is the OpManager Agent?

I may be a bit thick here; but I see no mention of the "ManageEngine OpManager Agent", but as I can see, it is an installable; and I have it running as a service on a few servers (Windows 2008 R2 all of them).

What is this? And what does it give me that WMI doesn't? And why is it barely mentioned anywhere?