Aluminium as a construction material has considerable advantages over other, more traditional materials. Firstly, fundamentally, it is by far the most plentiful metal on Earth, where it occurs naturally in oxides, and it is the third most abundant element on the planet after oxygen and silicon.
Aluminium rainwater systems compete with wood, cast iron and plastic, and have advantages over all of these. Those traditional guttering systems have joints, which are potential weak points that impact on longevity and cause water ingress and damage to the property, whereas seamless aluminium has no joints in the running length. Plus, seamless aluminium gutters are cut to the precise dimensions of the building, so there is little waste.
Those other materials all have weaknesses that aluminium does not: aluminium does not rot like wood, rust like cast iron, and it is much stronger and less prone to damage than plastic. Aluminium melts down easily to be recycled into other products, and the fact that aircraft built in the 1940s are still flying attests to the durability of aluminium.
Plastic gutters are harder to recycle and tend to end up in landfill, and few can have missed those tragic pictures of plastic waste washing up onto beaches or afflicting marine life because of plastic does not readily break down in the environment. Seamless aluminium gutters come factory coated, so, unlike cast iron and wood, there is no need for painting. Also, plastics are derived from oil, with all the attendant problems that implies. Aluminium is the green choice, the best option for the environmentally-aware.