Well, besides being able to see, which this thread addressed...
Being seen is also a big problem.
A single headlight does not allows cagers to judge your oncoming speed
and distance well, slow down or risk getting cut off.
In city traffic, the single headlight can be mistaken for one of the headlights
of the car behind you, making you completely invisible at times.
The single tail light on most bikes has the same problem, getting rear-ended
at night is a big risk for bikes. Only the big tourers with the lights
mounted on hard luggage overcome this well.
So, watch your six.
Or you can go crazy like I did and install lots of extra lights,
I have to commute after dark, so I lit up my bike just like a car.
Daytime running lights on front (ye old light triangle),
and additional running/brake lights on the back, as big and bright as I could mount.
Doesn't help with deer, but does wonders with cagers.