They make it harder to confirm the selected function and Off position. Guessing from a neighbouring knob can lead to the wrong mode.
What the observation means
Strong cleaning chemicals and abrasion may remove printed markings while leaving the knob mechanically usable. A handwritten replacement mark can be misleading if the control or knob has shifted. Correct identification requires the exact model and compatible parts or approved markings.
A useful next step
Stop using selections you cannot identify confidently. Keep a photograph of the remaining panel and the model reference for assistance. Do not test unknown positions by heating the cavity or assume every blank mark corresponds to a harmless fan setting.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.