Re: line to leader connection.
As explained by Terry Gunn, Lee's Ferry Veteran Guide:
"Loops hang up in the guides; I've seen thousands of fish broken off because of that and countless rod tips broken when people attempt to pull out stuck leaders.
Loops create a hinge between the leader and fly line when casting, add more mass and can be wind resistant.
Loops can create a point of attraction for leader and tippet and can create a knot that would have otherwise slid over a clean knot.
You can tie a nail knot (tie fast nail knot tool) or a surgeons every bit as fast or faster as you can a loop knot and end up with a clean and continuous connection.
Loops catch fisherman not fish."
Couldn't say it better myself.