What and why need fishing leaders when you going to fishing?

I know that some anglers get frightfully overexcited about this - but leader or no leader? (the lads in Guernsey got me thinking about this again) — Henry Gilbey

FLY FISHING FOR CARP: COMPLETE GUIDE - ToFlyFish

Fish Finder Rig for Surf Casting and Still Fishing

SS7732 Big Game Hook-Mustad Style -Terminal Tackle-TERMINALPRO

How To Store Fly Fishing Leaders? – MUSKEGON RIVER FLY SHOP

Aluminium Double Crimp Sleeve - Commercial Longline Fishing-TERMINALPRO

3.5 4.5 5.5 6.5 hollow swimbaits - Soft Plastic Baits-Terminalpro

How to Tie the Best Wind-On Leader

Fly Leader Length & Strength