What are the advantages of combiner over reducer or why do we need combiner when we are using same reducer class as combiner class ?

The main purpose of Combiner in Mapreduce frame work is to limit the volume of data transfer between map and reduce tasks.

It is a general practice that same reducer class is used as a combiner class, in this case, the only benefit of combiner class is to minimize the input data to reduce phase from data nodes.