The ESTA is another legal entry permit that differs from the non-immigrant visa (B1/B2). Depending on the applicant’s nationality and aptitude status, one of the two permits will have to be presented to the customs and border guard when entering the United States.
B1/B2 visa holders do not need to submit an ESTA application, as a valid visa meets all the requirements of American Customs and Border Protection and is sufficient to pass through passport control. However there might be reasons why you still might want to apply for an ESTA. If your B1/B2 visa has only a limited validity left, it might make sense to apply for an ESTA and present it when you enter the USA. Furthermore it could be just as well that your passport expires soon. In this case, applying for a new passport would also make sense, this would mean that you would also need a new entry permit. As it is much easier to apply for an ESTA, it would make sense to apply for an ESTA even though you still have a valid B1/B2 visa at the same time.