Councils have certain duties which they must perform, and certain powers which they may exercise, pursuant to the Local Government Act 1999 as well as a range of other Acts. In most cases the relevant Acts confer those obligations and powers directly on the Council as a body.
It is not practical or efficient for the Council as a body of elected members to perform the many functions or undertake the many activities that are required in the day to day administration of the Council's roles and functions. Delegations are the way in which the Council enables other people/bodies (usually Council officers) to undertake these steps on its behalf.