driving in heavy rain on the highway, your forward visibility can be drastically reduced by the spray generated by the cars you are following. A rear foglamp is a great help in such situations, as in ... thick fog (!). The spray or fog create glare which reduce your forward visibility. So what i do is put on my Serengetis, and this gives me that oh-so-critical extra few yards of visibility.
i sometimes use the front foglamps at night when i am in a residential area, so i can see better anyone who is attempting to cross the street.
the front fogs are also useful for night driving on winding or zigzag roads, because they illuminate the bends in the road, so you can hit your apexes spot-on. Endurance racecars have driving lamps aimed to the sides for this purpose, also.
because my car is a 1989 model, it does not come with daytime-running lamps (thankfully!). I turn on my front foglamps in overcast or rainy daytime conditions, and these are excellent for being seen because of the wide beam angle.
and by the way, i like the Benz (European?) convention of having brake lights that are separate from the tail lamps, to make night braking more distinguishable to the other drivers. The 3rd high-mounted brakelight helps, for those taillamps with brake lights in the same cell as the taillights (i.e., with double-filament bulbs). The worst designs are the American taillamps, which do not even use amber for turn signals, and blink a group of lamps for this purpose.
now, if i could find a way to (tastefully) add front driving lamps to my 190e, without buying those expensive airdam kits ... that is, without making it look like one of those riced-up hondas ...