You can determine the offset via a database of cd/dvd drives (via accuraterip website), or by using a known cd in accordance with EAC/dbpoweramp instructions.
As to why,
http://www.exactaudiocopy.de/en/index.php/support/faq/offset-questions/
http://www.dbpoweramp.com/cd-ripper-setup-guide.htm
Most people who rip for archival purposes use the proper offset, which is one of the main advantages of EAC/dbpoweramp.
As to why,
http://www.exactaudiocopy.de/en/index.php/support/faq/offset-questions/
http://www.dbpoweramp.com/cd-ripper-setup-guide.htm
Most people who rip for archival purposes use the proper offset, which is one of the main advantages of EAC/dbpoweramp.