If you don't really know what it is, then it is best to leave it on.
If you understand what it is and have the ability to keep things in proper adjustment, then take it off.
There used to be some really heavy chrome-plated steel ones which were great for helping get up to speed downhill.
If you really are a dork, then you are obligated to leave it on.
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer