Mammogram: benign or not?

 I went to ge tiny firs tmammogram last week because I had discharge. they found dense tissue so they had a diagnostic mammogram done today.  they also did an ultrasound and they found some masses that are benign. The doctor said since there is no blood flow inside of them, they’re nothing to worry about. However I walked away wondering “what is the difference between a benign mass that you have to remove and one that you don’t have to remove?” Was wondering if anyone knew. Thanks! Also how common is it to misdiagnose? I know someone who had a similar experience and the following year they did a mammogram and had stage 4 cancer.