The only real advantage is keeping the hose's from blowing out. Other than that, it's just for looks.
less flex in the tube/ line when you brake so the force is pushed into the brake caliper as opposed to inflating the brake hose.
steal
Aluminium is lighter and doesn't rust Steel is cheap, it is very hard, but can break
that plastic is cheaper to have on a car and it would hurt someone you crashed into less than a steel car would
it transports you and family if you live far away from work or shopping, other than that, there are many disadvantages.