There are many reasons. Ravana was given a boon by Brahma that any being, other than a human or animal, will not be able to kill him. To jump and fly over the ocean from India to Sri Lanka was not possible by a human being.
Also, Brahma had requested all Gods and Goddesses to be born as monkeys or bears to come and help Sri Ram in his quest to defeat Ravana.
This could also be a reason for Shiva to be born in the form of a monkey to help Vishnu who was born in the form of Ram. Also, Ravana was cursed by Sri Nandishvar when he laughed at his looks, and he cursed that his death will be in the hands of someone who will be in form of a monkey. Also, animals do not need any homes or cooked food.
They can survive in the jungles, which becomes a benefit during a war. Moreover, Hanumanji was able to do a lot of 'lilas' since He was born in the form of a monkey. For instance, He does not need a vehicle to cover long distances in the search of Sita, nor he could jump over the ocean when required, nor would he been able to burn Lanka if he did not have his tail. Finally, it teaches us an important lesson. Whatever form you may have in this world, if your deeds are pure, than your life becomes worthwhile and an shining example for everyone.
