The original purpose of the overhead join was to allow either non-radio aircraft, or aircraft arriving at a non radio airfield, to overfly the airfield at a safe height, to observe the signal square, determine the runway in use and circuit direction, and then descend into the circuit pattern
which dates back to WW1
The OHJ is dangerous because - ignoring for the moment the 'people' who insist on the OHJ even when there is no known traffic, just to wind up visiting pilots (a certain airfield in the Luton area being one well known example) - it gets invoked when there are too many planes inbound at the same time, which is precisely when you do NOT want to be heading for the same spot at the same altitude!
The OHJ is also used at ATC airfields when the ATCO cannot handle the incoming traffic so he just sends everybody into the overhead and people then have to take care of themselves. And one finds some people cheating. For example the other day I arrived in the overhead, only to hear a certain other plane which was way behind me call "overhead". He got what he wanted (the clearance to descend deadside) ahead of me as a result. I wasn't bothered (never would have thought of that trick myself, but then I am not paying a club hire rate) but it shows the system is far from foolproof; the first person to call 'overhead' is likely to get a descent clearance regardless of where he actually is.