Assuming that you have asked an honest question , i am making an attempt to clear your doubt. Rama, being a god did not require any support to get his work done. But, in this avatar he took a vow that he would be human in all ways and would stick to dharma, satya etc. He could have killed Ravana from indian shores but he did not choose to do so. Apart from various other objectives of HIS avatar he also wanted to show mankind how to lead a life on the right path.Devas wanted to contribute and help rama in his noble endevour to anhilate evil. They were born as vanaras. So he took their help. Now regarding the bridge , there were several instances to prove the requirement of the bridge inorder to show many important lessons which occured during the building process, like- destroying sagar raj's ego, demonstrating nala's skills, demonstrating the power of Rama Nam, Clearing Hanuman's doubt, Squirel's contribution, The power of faith so on so forth...
