Why is the diameter of the front element of any importance?
Theoretically the larger the front element of the binoculars, the better the image result will be. The reason for this is that an increase in the front element’s diameter will increase the light-gathering capabilities of the binoculars. It should, however, be noted that there are other factors that influence the light-gathering capabilities of the binoculars such as the refractive index of the glass and coatings used to reduce this figure but can be ignored for now.
A larger front diameter will also give you a larger exit pupil (if magnification is kept constant). The exit pupil size indicates the size (in diameter) of the light beam exiting the binoculars at the eyepiece. Larger exit pupils are preferred as the ease of viewing is improved; less “dark circled edges” are visible in low light conditions and even eliminated during good light conditions. The reason for this is that the exit pupil is smaller than your own pupil when viewing through the binoculars. So, simply, there is no light on the outer edges of your pupil and thus the dark areas. Increasing the magnification of a pair of binoculars will decrease the size of the exit pupil and decreasing the magnification will increase the size of the exit pupil.