The reason appears to be that for some the bulbs supplied are not bright enough or should be more white in colour. Some especially do not like the yellow indicator bulbs. Personally I find the bulbs supplied sufficient but if some want different bulbs let them have them. The only thing that irritates me is those who choose to use very ultra bright bulbs and then drive so close that my car is filled with light and I am dazzled by them.
I live in the country and with no road lighting at all, the stock headlights are just too dim....I've osram nightbreaker ultimates as headlights...huge difference in light throw and brightness whilst still being legal and a relatively cheap upgrade at around £13 the pair.
Indicators...I disliked the big orange bulbs at the front so switched to phillips silvervision....makes the car look better imho
DRL lights, well once you upgrade the main lights, the drls/sidelights look very yellow in comparison so some folk switched to MTEC bulbs (I did but they lasted only 2 months (5k miles for me) I now have LED bulbs fitted but keep a set of standard filament ones in the car in case the boys in blue have any issue with led DRL's (and to switch when its time for MOT)
hope that explains the rationale behind switching bulbs
The LED DRL lights are also sidelights and I'm not going to risk LED sidelights for the mot...they are clearly not as omnidirectional as the original filament ones and the 5w setting is just as bright as the 21w drl setting.....the indicators and headlamps are definitely MOT Legal.
To be honest, I think the Mtec bulbs look great but my silly mileage means they have the equivalent of 1 yrs use in 2.5 months. I bought a box of 10 ordinary dual filaments for £16 at the autofactors...!