I had 2abnormal mammograms , an ultrasound and a core vacuum biopsy of left breast , which was benign..why do i need an mri?

Please ask your doc: Most likely they saw another area and they want to get a better look but please discuss with the radiologist or get a second opinion form a breast speciialist that is not a radiologist like a breast surgeon.

Very sensitive: Sounds like you doctor does not like the look of the mammogram and is concerned that the other tests are missing cancer. The MRI is very good at picking up cancer. If its negative, then the mammo is likely false.

To not miss a cancer: The needle biopsy pathology report may not explain the abnormality on the imaging studies and radiologists are trained to do additional studies or repeat the biopsy if not satisfied with the pathology report. This is called "discordance" i.e. The mass is too worrisome to accept a negative report. The needle biopsy may have missed the mass. A negative MRI may prove that the path report is correct.

It is really: difficult to speculate. Were the benign biopsy results discordant with(i.e. they did not explain) the mammographic finding? If so perhaps the MRI is being performed to increase the level of confidence that there is no cancer. But that is just a guess, you should talk to your doctor about it.