The AS override feature can be used in VPRN scenarios where a customer is running BGP as the PE-CE protocol and some or all of the CE locations are in the same Autonomous System (AS). With normal BGP, two sites in the same AS would not be able to reach each other directly because there is an apparent loop in the AS path.
When override is configured on a PE-CE EBGP session the PE rewrites the customer ASN in the AS path with the VPRN AS number as the route is advertised to the CE.