Most ribbon microphones are passive devices – they have no onboard active electronics or preamplification. Because of this, the impedance of the preamp input to which they are connected is critical to the sound produced by the mic. If the impedance is too low, frequency response will change, particularly on the bottom end.
But before you assume this is a bad thing, it can result in useful sounds. In fact, you can almost think of it as a low-frequency EQ option. For example, let’s say you have a passive ribbon mic on a source, and because of proximity effect or some other factor, the sound is too bassy. You could reach for an equalizer to tailor the sound. Or, you could plug the mic into a preamp with variable input impedance and lower the impedance setting. This will reduce the low-end response of the microphone and may result in exactly the sound you want or need.