Current limiting or voltage dropping
Two things. First, Vf is not a static number. Vf changes with If. Vf changes with temperature. Vf varies from one LED to the next, even if you do your comparison at the same temperature and same If.
What makes this particularly troublesome is that a small change in Vf can cause a large change in If. So if you are building 10,000 devices, and trying to drive an LED with a specific voltage, some of them will get too much current and fail in seconds or minutes or hours or days. Some will get too little current and be too dim, and maybe, some might actually work OK for a long time (or maybe not).
So the basic idea with LED's is that you must control the current rather than the voltage to get acceptable results over the long run and across production variation. So, in my opinion, the best terminology for the resistor in series with the LED is &quot;current limiting resistor.&quot; It would also be OK to say &quot;current setting resistor.&quot;
By placing a resistor in series with the LED you are approximating a current source (assuming the voltage is fixed... if the voltage varies, it may be better to use an actual current source of some sort). Hope that helps you think about this in the right way. You will for sure find people (usually novices) who get away with driving LED's without a current limiting resistor in certain circumstances for various reasons. But it is best to have a well defined limit that you choose as part of a design process to avoid surprises.