in the Old Testament, this was a very ambiguous term:
it could mean 'I', a human being, a representative of humanity or a supernatural figure bringing god's judgement
it was Jesus' preferred title for himself- its ambiguity made it less likely that he would be thought of as a power figure and potential revolutionary
he used it to describe his role as the 'sufferingservant' spoken of in the Old Testament and to his god-given authority both in the present and the future
it also humanises him, making him seem to christians that he is the same of them, sharing their suffering