These days,most cars come equipped with fog lights standard,so this might seem a bit of a useless question. Still,a good question and i will try to adress it from the average person point of view.
Normally,fog lights are meant to be a help when driving in foggy days,or rainy or snowy ones.But let's be honest- as a fog helping tool,they do crap.Sometimes i wonder if they really are on,because when fog is really thick,nothing can really go trough.
The only real advantage,worth having fog lights,is that you get noticed more and better by other drivers,and this is no minor thing.Especially on highway,having four lights instead of just two,is a heck of a difference,believe me. But normal fog lights have normal light bulbs,that get average light,and burn quite often,if you use them daily.
Why some fog lights seem to stick to that old-looking yellow light color? Well,it's just that a theory,proven false,said that yellow light has a different wave length and so penetrates better the fog. As i said,proven false,end of story- just stick to white light,already standard on new car models sincer 2000+. What makes thing even worse,yellow lights usually have an optical filter that actually takes a percentage of light,so you even get less light in those hellish foggy days.
For this reason,many car manufacturers introduced driving lights. What are these? A set of LED (light emitting diode) lights that can be used continuously without increasing fuel consumption,and they get burned after tens of thousand of hours of use. Bein a LED,they offer a brighter light,and in return,you are more visible on the road.
Some car manufacturers,offer fog lights with an interesting function,adaptive cornering function,that makes the fog lights turn as you turn the steering wheel.Pretty neat,i'd say,especially at night.One of these vehicles is Fiat Bravo,an European car.For this,please review the next link http://www.fiat.com/FIAT_COM/uploads/PB_TMPL_SHOWROOM/1073903335/20070315/Bravo_catalogo_BlueMe_UK.pdf
So,we can state that the vast majority of fog lights are purely aesthetic in nature,but newer version are better although still hardly effective in foggy days.
No matter what you choose,fog lights or not,if you plan on using them normally,get them mounted at a professional car mechanic,so you don't burn your alternator,or something else.
Periodically,check if they are correct aligned,as bumps on the road can affect the screws.
Also,if you need a more scientific opinion regarding the difference between yellow or white color fog lights,be sure to check this great site,Newton BBS:
http://www.newton.dep.anl.gov/askasci/phy99/phy99xx4.htm
