Hi David;
I'm old school as well - buts lets start somewhere:
1. Leaders used in Surfcasting have been around for years and tend to be shock leaders - about 8m of heavy line to guard against breakage when making long casts from the shore.
2. Mono or fluoro carbon leaders are necessary when fishing with braid as they are more transparent to the fish - having scuba dived I can tell you that braid is so easy to see in depths of up to 35m that a 5-10m length of leader on the end of braid will ceratinly help.
3. Braid fishermen get very sensitive bites but also use leaders of mono when fishing over reefs/wrecks etc that can cut braid very easily. Imagine using a 2m leader off braid when floatlining - you would still have 5m or so of braid in the reef or floating very close.
4. Because mono is so thick, most leaders are now made of fluoro carbon and generally the supple variety. You can combine a leader off braid of say 5+ m of mon and then use a short 1m FC leader in your terminal rigs. This way you can change rigs/weights etc quickly without messing about with the long length of mono at the end of your braid. It is also cheaper to use part mono as well.
5. Double leaders are used in IGFA fishing and the length and type is regulated to facilitate handling offish close to the boat etc. Lure fisherment also use doubles for similar reasons.
Anyway - thats a start and i'm sure lots of people have different reasons/uses as well. I also use FC leaders of up to 2m when fishing all mono as the lower visibility can only help!
Cheers
Plato