In urban situations where there is more ambient light / pollution a slightly brighter light is needed to overcome that so you can see oncoming hazards and make yourself visible to oncoming vehicles, cyclists, and pedestrians.

Get out in the country and you can actually do with much less output.

Many European laws require a 3 watt headlight which is more than enough to see with and enough to be seen and well designed lights have a cutoff and don't direct light up into the eyes of oncoming persons.

Same goes for rear lights... consider that you need to be seen but you also have to consider who is following you.

In many cases the lighting of one's bike has become a "who has a bigger johnson ? " contest and some people are downright stupid when it comes to lights as they run lights that are too bright for cycling and aim them very poorly.

I run a 2w PB headlight and run the strobe in the daytime as it is visible from great distances... on high it has enough output to overcome most ambient light and can run on low in the dark and see just fine.