It would sometimes connect to an RF screening can, or possibly connect to some screening parts when it's assembled into the case. It looks like there make be continuity to one of the pins on the device at the top right. If it's not connected to ground, it would be a risk for EMC.
I agree that it could be something to reduce electromagnetic interference, or it could be to help ground a case or something else like that, it is a bit difficult to tell without knowing if its connected to anything else or is just a ring by itself.