Reason a) Golf is synonymous with a form of fashion, to play Golf you must be wearing the 'correct' (i.e. insanely expensive) clothing. Some Golf courses have a high required standard of dress and although not breaking the rules, non-Golf specific gear is frowned on. Other courses are more relaxed and as long as you don't wear your crampons on the green they don't care.
Reason b) A Golf jacket is likely cut differently to minimize the effect on the swing. At sub 20 handicap it makes a difference if you swing is altered by wearing a jacket. Starting out, there are more important things to worry about and the jacket will not make a measurable difference to your game.
If you are a cyclist this is exactly same as the Lycra requirement on road bikes. Once you reach a certain level you splash out on the specific gear the 'rules' of the sport dictate you must have, so you fit in and have the gear to maximize your enjoyment of the sport.