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Always thought it was (combined with those ears that stick out of the inside of chainset) to stop the chain jamming in the gap between BB spindle and inner ring.
You can pull them off if its causing problems, just a press fit.Edit, talking of shimano UN300, had a weird one of the other day, screw threads on the drive side stopped about 5 full threads before the collar with a solid chunk of alloy (full height of the thread inside of the valley height), ended up taking a burr grinder to it. BB axle clamped in vice, then just let it spin like a kind of master bodge lathe setup. BB has plenty of threads on it, and they usually aren't threaded all the way upto the collar. Was a 122/123 x 73mm, never seen that before.
BrickMan
Had one in front of me. Pried it off a bit and you can see it butts up against the machined section on the spindle, it's made of steel and too far away from the bearings to be of any use as a seal or dust cover. They're there to stop you installing the crank too far on, you may not need it if you reach installation torque before they contact but if you don't it'll stop them going on too far. Most BBs don't have them and I've always thought they're a bit weird.