As Caregiver said, it is probably to make sure the biopsy found all it needed to find. I would sure ask your physician this question, he should be able to give you the answer you need.
In all liklihood the first sample was inconclusive.
It wasn't that they did anything wrong. This just happens.
They want to make sure there is a requirement for surgical removal of a malignancy, for example, before making such a recommendation. Absolutely positively certain.
This is a difficult question to answer as you haven't provided much info. However, if you have any concerns about needing a second biopsy you need to bring this to the attention of you doctor, or specialist, whom ever ordered your biopsy in the first place. Asking questions about your health is your right, keep asking until you understand what it is the doctor is doing. All the best to you, Please, keep us in mind if you need us.