Some hooklength materials in lower breaking strains tangle easier.
Higher breaking strains/stiffer material such as fluorocarbon because of the anti-rejection properties and hooking effects.
I`ve used braid in 4lb,7lb & 10lb and it`s ************e. It just unravels itself during a session, but in 15lb it`s fine.
So in a nutshell - desired effect and presentation.
Also, the rod and mainline cushions the fish lunges etc, but a short hooklength of lighter material will break before the mainline.