Originally Posted by
dddd
If the crankarm settles inward far enough, the front derailer may not retract inward far enough against an oversized seat tube to effect robust shifting down to the smallest chainring, hence the collars came into being to prevent crankarms settling inward and front derailers no longer quite doing their job.
If that bit of soft plastic stops the crank from being fully tightened, you'll be buying a new crankarm.
So if it's used (in the situation you describe) to do so, the axle is too short.