Contrary to its name, an outstanding arrest warrant is not a good thing.  It is simply an arrest warrant that has not been served on a person.  A person might be evading law enforcement to prevent the warrant from being served or the person might not even know that law enforcement is looking for him.  Additionally, there might be a backlog of warrants to serve and therefore a law enforcement agency might have a lot of outstanding warrants.

When police pull over a vehicle, they typically search a warrant database to see if the suspect has any outstanding warrants.