Applications that rely on ambient light or need short exposure times are typical examples of the need for good sensitivity.
There are mainly two parameters influencing the maximum sensitivity of a camera: The quantum efficiency which indicates the percentage of photons of a specific wavelength that are converted into electrons. Modern sensors reach a maximum efficiency greater than 60 % for certain wavelengths. The second important parameter is the background noise that can be measured without light hitting the sensor. Consequently a sensor is more sensitive with a higher quantum efficiency and lower background noise.
The sensitivity is often mistaken with the maximum brightness of an image using controlled illumination. This apparent brightness, however, can also be reached easily by using a grey value multiplication, often called gain. The disadvantage of using a higher gain multiplier is increased noise and no additional signal. As the sensitivity of a camera is normally specified under different conditions and in a different way by the manufacturers, it is sometimes difficult to compare true sensitivity.