I have the patagonia guide jacket--no I didnt pay full price for it. I mean it is borderline ridiculous to think of someone paying $400 for a jacket like that, but it is a pretty impressive garment. Absolutely nothing to snag fly line(you wouldnt believe how many straps and zippers and crap that are on a regular rain jacket like north face or marmot etc.) Beefy pockets up front for fly boxes, cameras, and other gear you dont like getting wet. A soft and plush colar lining for when you have the hood up and the jacket fully zippered. This colar lining is especially nice if you have had a plastic zipper brushing up against your mouth like on other jackets. Finally, the materials used are exquisite.
Ok, onto the "why?". First, do you fish in the winter? Having wet anything when its below 40 out just plain sucks. It is easy to slip and put your arm in the water, or release a fish and get your forearm wet. Wading jackets have a cuff that fits pretty snug(usually made of neoprene or another waterproof material) and this prevents water moving up into the jacket. If you wear the jacket + waders + waterproof gloves(pata's R1 glove for example) you can essentially go for a swim at some point during your fishing trip and still stay dry. This is my steelhead fishing outfit, and I am mighty comfortable. All of this gear exists for a reason--do you want to have your fishing be determined by the elements, or do you want to get out and fish despite the conditions? PA weather sucks for the most part, so I am the latter.