Broke part of rotor when replacing wheel bearings, what does it do?

Hey guys,

So I was replacing my wheel bearings on my 94 explorer front driver side; when pulling out the wheel bearings I snapped this:

http://i.imgur.com/tvwyP.png

It was on there completely snug before and once I snapped it, it's loose and pulls right off the rotor. I drove it a short distance to park it, and in that time it made an annoying clunking noise (didn't sound dangerous at all, just sounds like its on there loose).

What's this part do? It doesn't look like those teeth grab anything, so I'm just curious as to what it does?