Sy Heres some stuff on the simple answer
for a shockleader.
Shock leaders are the length of your rod plus one half. anything more than 20 feet will disqualify you for any IGFA records.
Heavier line on the last 5 feet or so are for these reasons. (generally it is from 18 inches to 5 feet. (I use 30 inch leaders.)
1. For abrasion protection against rocks and muscle beds.
2. Stealth when using flourocarbon as your leader. The light refracts very close to the index of water with flourocarbon.
3. Stretch. Monofilament will stretch a little bit. This is useful when using braided lines. Tying on a mono leader will lessen the hook being yanked from the fishes mouth on a hook set.
Hope this clears it a little bit.