That i can answer because i've been reading/seeing a lot for a /film/ thread.
>some Chinese dao
The ringed swords have another name which i don't recall at this moment, they were significantly wider than a normal sabre (Dao) and had to be attached to a "broken" disposable sheath meaning it's considered more of a military sabre/cleaver than anything, used more for chopping someone rather than striking/thrusting vital points in the traditional dim mak thing both chinese sides had in their martial world aka it's more for soldiers/butchers than royal arms.
>were ring dao even an actual traditional weapon
They were real but quite rare because sabres were not that used by the ones who could pay for their work aka royal forces, who liked the Jian-type blades more due to muh chivalry reasons. Only rich warlords who disregarded maiming-for-honor in favor of straight killing used sabres aka the Yuan/mongols, some mongolian cleavers in museums do have a ring or two so it might be an adaptation from them although i doubt they were used on field other than cutting livestock.
>or are they just something made up by the HK movie industry?
Shaw Brothers movies liked to use it a lot because of its supposed symbolic use, which was:
>What was the purpose of the rings
In almost-certainly fictional utopia boxer world lore the rings are used to "signal" the wielder's presence, that means a fighter who is willingly forfeiting the advantage of striking from behind without the opponent knowing, which also means that his entrance/style stance uses the ringing of them too for ritualistic/bragging reasons. Overall it just means someone who is frontal and "honest" in his fights, unlike someone who might have an easily-concealable weapon painted black like the Ninja.
the actual, suspected real reason of them is to "catch"/"jam" thin profile bladed weapons when they approach in frontal thrusts, like the Jian (flat fencing sword), the Ji (dagger polearm) or the Qiang (spear), somewhat akin to what a spear's tassel would probably do if rolled around another spear. Because the weapon is also heavier and thicker, if you hypothetically jammed something with it you could break or bend it if enough pressure is exerted. Of course this is all a supposed because old ringed dao are specific custom-made things rather than standardized tools, probably from a short-lived obscure martial clan or a couple of eccentric swordsmen.