An auxilairy bishop is not the ordinary bishop of a diocese like a regular bishop or archbishop is. (An archbishop is the head of a larger see - such as Chicago - and also has a certain degree of oversight responsibilities in his larger provincial area beyond his personal diocese.) The auxilary bishop is an assistant to the ordinary in a diocese which requires more episcopal service (confirmations and all). Most major dioceses have some auxiliaries, therefore. The Vatican does like to limit the number a bit and will get upset if the ordinary asks for too many assistants. But if the diocese can justify their service, then the pope will appoint them. Since a bishop does need to be, by his nature, the head of a diocese, these auxilairy bishops are appointed as titular (basically honorary) heads of historical dioceses which no longer exist as established ecclesial comunities.
