I had a shave biopsy on a mole and my dermatologist told me that i need a excision biopsy. why?

Malignant potential: Depending on what the pathology report showed your doctor wants to remove all of the mole not just the surface . It is possible that is looked like it could be a skin cancer that is something you MUST discuss with your doctor and have the rest of the mole removed.

Residual lesion: A likely reason is the lesion was present at the resection margin of the shave biopsy. It is generally important to evaluate the interface between a lesion and normal tissue.