Virtual sequence/virtual sequencer

What is advantage of using virtual sequence over normal sequences. Can anyone help me with where exactly we need virtual sequence/virtual sequencer.

In reply to kranthi445:

A seuqnce desribes how sequence items/transactions will be created.
A virtual sequence does not cretae any sequence items/transactions itself. It is used to orchestrate the execution of several sequences.
I’d recommend to read the corresponding chapters in the UVM Cookbook.