Question

why do we compare arteries like tree which divides into smaller and smaller branches

Answer 1

As the tree have so many branches likewise the arteries also divided into smaller and smaller branches.Blood supply through these arteries to all the body parts. so, to reach all the body parts of the body , it divides into number of branches to supply blood just like as branches that supply water and food material  to other parts of plants.
So, we can compare arteries like tree which divides into smaller and smaller branches.