Aluminum Bonsai wire is used to shape branches of your tree. You could compare wiring a tree to an artist using a paint brush to paint a painting. Young trees grow their branches towards the sun naturally and then over time as they start to get heavier and with age, the branches start to slowly bend down to a more horizontal position.
In Bonsai we use aluminum wire to give the trees movement, we apply the wire and bend the branches into position, then we wait until the branches start to expand and remove the wire as soon as it starts to eat into the branches. Wire has to be applied numerous time to keep the branches from losing shape. It is very difficult to create bonsai without any wire, by using wire you can shape the canopy of a tree to look like an alpine style tree or a windswept or how about a big Banyan style Ficus? By using wire, you are the limit of your own imagination.